Cylindrospermopsin toxicity in mice following a 90-d oral exposure.

Cylindrospermopsin (CYN) is a toxin associated with numerous species of freshwater cyanobacteria throughout the world. It is postulated to have caused an episode of serious illnesses in Australia through treated drinking water, as well as lethal effects in livestock exposed to water from farm ponds. Toxicity included effects indicative of both hepatic and renal dysfunction. In humans, symptoms progressed from initial hepatomegaly, vomiting, and malaise to acidosis and hypokalemia, bloody diarrhea, and hyperemia in mucous membranes. Laboratory animal studies predominantly involved the intraperitoneal (i.p.) route of administration and confirmed this pattern of toxicity with changes in liver enzyme activities and histopathology consistent with hepatic injury and adverse renal effects. The aim of this study was designed to assess subchronic oral exposure (90 d) of purified CYN from 75 to 300 µg/kg/d in mouse. At the end of the dosing period, examinations of animals noted (1) elevated organ to body weight ratios of liver and kidney at all dose levels, (2) treatment-related increases in serum alanine aminotransferase (ALT) activity, (3) decreased blood urea nitrogen (BUN) and cholesterol concentrations in males, and (4) elevated monocyte counts in both genders. Histopathological alterations included hepatocellular hypertrophy and cord disruption in the liver, as well as renal cellular hypertrophy, tubule dilation, and cortical tubule lesions that were more prominent in males. A series of genes were differentially expressed including Bax (apoptosis), Rpl6 (tissue regeneration), Fabp4 (fatty acid metabolism), and Proc (blood coagulation). Males were more sensitive to many renal end points suggestive of toxicity. At the end of exposure, toxicity was noted at all dose levels, and the 75 µg/kg group exhibited significant effects in liver and kidney/body weight ratios, reduced BUN, increased serum monocytes, and multiple signs of histopathology indicating that a no-observed-adverse-effect level could not be determined for any dose level.

A critical review of the postulated role of the non-essential amino acid, ß-N-methylamino-L-alanine, in neurodegenerative disease in humans.

The compound BMAA (ß-N-methylamino-L-alanine) has been postulated to play a significant role in four serious neurological human diseases: Amyotrophic Lateral Sclerosis/Parkinsonism Dementia Complex (ALS/PDC) found on Guam, and ALS, Parkinsonism, and dementia that occur globally. ALS/PDC with symptoms of all three diseases first came to the attention of the scientific community during and after World War II. It was initially associated with cycad flour used for food because BMAA is a product of symbiotic cycad root-dwelling cyanobacteria. Human consumption of flying foxes that fed on cycad seeds was later suggested as a source of BMAA on Guam and a cause of ALS/PDC. Subsequently, the hypothesis was expanded to include a causative role for BMAA in other neurodegenerative diseases including Alzheimer's disease (AD) through exposures attributed to proximity to freshwaters and/or consumption of seafood due to its purported production by most species of cyanobacteria. The hypothesis that BMAA is the critical factor in the genesis of these neurodegenerative diseases received considerable attention in the medical, scientific, and public arenas. This review examines the history of ALS/PDC and the BMAA-human disease hypotheses; similarities and differences between ALS/PDC and the other diseases with similar symptomologies; the relationship of ALS/PDC to other similar diseases, studies of BMAA-mediated effects in lab animals, inconsistencies and data gaps in the hypothesis; and other compounds and agents that were suggested as the cause of ALS/PDC on Guam. The review concludes that the hypothesis of a causal BMAA neurodegenerative disease relationship is not supported by existing data.

The course of toxicity in the pregnant mouse after exposure to the cyanobacterial toxin cylindrospermopsin: clinical effects, serum chemistries, hematology, and histopathology.

Cylindrospermopsin (CYN) is a toxin produced by a variety of fresh-water cyanobacterial species worldwide and induces significant adverse effects in both livestock and humans. This study investigated the course of CYN-induced toxicity in pregnant mice exposed daily during either the period of major organogenesis (gestation days [GD] 8-12) or fetal growth (GD13-17). Endpoints include clinical signs of toxicity, serum analyses to evaluate hepatic and renal function, histopathology of liver and kidney, and hematology. Study animals were administered 50 µg/kg CYN once daily by ip route and euthanized 24 h after 1, 2, 3, 4, or 5 consecutive doses, or 6 or 13 d after the dosing period. The course of the CYN-induced effects was determined at all euthanasia times for the endpoints just outlined. Results indicated that CYN is a toxin, producing lethality in dams during the early part of gestation, significant weight loss, and bleeding in the gastrointestinal tract, tail tip, and peri-orbital tissues. Effects also included alterations in serum markers for liver function, histopathological changes in liver and kidney tissues, electrolyte abnormalities, leukocytosis, and posttreatment thrombocytopenia and reticulocytosis. The onset of symptoms was rapid, producing reductions in weight gain in GD8-12 animals, bleeding in the vaginal area in GD13-17 animals, and significant increases in sorbitol dehydrogenase (SDH) in both groups after a single dose. Although the GD8-12 dams displayed a 50% lethality, in GD13-17 animals only a single death occurred. Alterations seen in hepatic and renal function or histopathology do not appear to be of sufficient severity to produce death. Evidence indicates that bleeding may play a critical role in the onset of symptoms and eventually, in the observed lethality.

Toxicity and recovery in the pregnant mouse after gestational exposure to the cyanobacterial toxin, cylindrospermopsin.

Cylindrospermopsin (CYN) is a tricyclic alkaloid toxin produced by fresh water cyanobacterial species worldwide. CYN has been responsible for both livestock and human poisoning after oral exposure. This study investigated the toxicity of CYN to pregnant mice exposed during different segments of gestation. The course of recovery and individual responses to the toxin were evaluated. Adverse effects of CYN were monitored up to 7 weeks post-dosing by clinical examination, histopathology, biochemistry and gene expression. Exposure on gestational days (GD) 8-12 induced significantly more lethality than GD13-17 exposure. Periorbital, gastrointestinal and distal tail hemorrhages were seen in both groups. Serum markers indicative of hepatic injury (alanine amino transferase, aspartate amino transferase and sorbitol dehydrogenase) were increased in both groups; markers of renal dysfunction (blood urea nitrogen and creatinine) were elevated in the GD8-12 animals. Histopathology was observed in the liver (centrilobular necrosis) and kidney (interstitial inflammation) in groups exhibiting abnormal serum markers. The expression profiles of genes involved in ribosomal biogenesis, xenobiotic and lipid metabolism, inflammatory response and oxidative stress were altered 24 h after the final dose. One week after dosing, gross, histological and serum parameters had returned to normal, although increased liver/body weight ratio and one instance of gastrointestinal bleeding was found in the GD13-17 group. Gene expression changes persisted up to 2 weeks post-dosing and returned to normal by 4 weeks. Responses of individual animals to CYN exposure indicated highly significant inter-animal variability within the treated groups.

Prenatal exposure to the herbicide 2,4-dichlorophenyl-p-nitrophenyl ether destroys the rodent Harderian gland.

Exposure of mice to the herbicide 2,4-dichlorophenyl-p-nitrophenyl ether during gestation produces abnormalities that are not readily apparent at birth but become obvious as the pups mature. By 2 weeks after birth there are severe intraorbital defects resulting from destruction of the Harderian glands behind the eyes. This effect is noticeable only postnatally because the Harderian gland does not grow or function until after birth.

The relationship of maternal and fetal toxicity in developmental toxicology bioassays with notes on the biological significance of the "no observed adverse effect level".

Standard developmental toxicology bioassays are designed to identify agents with the potential to induce adverse effects and include dose levels that induce maternal toxicity. The work reported here was undertaken to evaluate the relationship of maternal and fetal toxicity. It constitutes an analysis of 125 developmental toxicity bioassays in the mouse, rat, and rabbit conducted by the National Toxicology Program. Although varying by species, general findings include: (1) most lowest observable adverse effect levels (LOAELs) were determined by reduced maternal gestational weight gain or fetal weight at term. (2) Maternal weight reductions are associated with reduced food intake for a variety of dissimilar test agents. (3) Lower fetal weights were associated with reduced maternal weight gains late in gestation. (4) The degree of fetal weight reduction is correlated with the extent of the maternal weight loss. In a substantial number of the studies, reduced fetal weights at term may, therefore, be due to maternal undernutrition caused by general toxicity rather than direct developmental insult. Consequently, such test agents may be erroneously classified as primary developmental toxicants. Experimental approaches to test the hypothesis that maternal undernutrition in standard developmental toxicology bioassays may be responsible for significant term fetal weight decrements are discussed.

The cyanobacterial toxin, cylindrospermopsin, induces fetal toxicity in the mouse after exposure late in gestation.

Cylindrospermopsin (cyn) is a cyanobacterial toxin implicated in human and wildlife poisonings. We have completed studies investigating the potential of purified cyn to induce developmental toxicity in mammals. The teratology study involved intraperitoneal injections (8.0-128 microg kg(-1)) on gestational days (GD) 8-12 with subsequent examination of term fetuses for viability, weight and morphological anomalies. Cyn was lethal to a significant portion of the dams receiving > or = 32 microg kg(-1). Surviving pregnant females were killed and fetuses removed for examination. Analysis indicates no adverse effects on litter size, fetal weight, or incidence of anomalies. Subsequently, 50 microg kg(-1) cyn was administered on GD 8-12 or 13-17. Animals were allowed to give birth and litters monitored for growth and viability. A reduction in litter size occurred in treated groups. Avg. pup wt. was only affected in the GD 13-17 group. GD 13-17 dams did not exhibit the toxicity noted in the GD 8-12 group but gave birth significantly earlier than controls. There was a significant number of dead GD 13-17 pups and incidences of blood in the gastrointestinal tract and hematomas in the tips of the tails in survivors. Pups were cross-fostered to control mothers in litters of 10. On postnatal days (PND) 5-6 there were no significant differences in weight gain or viability in GD 8-12 litters, while GD 13-17 litters had significantly reduced weight gain and viability. GD 13-17 exposed male pups still weighed significantly less than the controls after 15 months.

Nicotine effects on the activity of mice exposed prenatally to the nicotinic agonist anatoxin-a.

Anatoxin-a is a nicotinic agonist produced by several genera of cyanobacteria, and has caused numerous deaths of wildlife, livestock and domestic animals world-wide. Several studies in the literature have shown that exposure of mice and rats to nicotine early in development alters its effects when the rodents are subsequently challenged with nicotine. We therefore determined the effect of nicotine on the motor activity of adult mice that had been exposed prenatally to anatoxin-a. Pregnant CD-1 mice received either saline vehicle or one of two doses of (+/-) anatoxin-a (125, 200 microg/kg), i.p., on GD13-17. As adults (8 months), control mice of both genders were used to determine the effect of nicotine (0, 0.1, 0.3, 1.0 or 3.0 mg/kg, s.c.) on motor activity measured for 30-min in a photocell device. Under these conditions, nicotine produced dose-related decreases in both horizontal and vertical activity, with an ED50 estimated to be 0.65 mg/kg. Next, additional control mice and mice exposed prenatally to anatoxin-a received the nicotine ED50 and saline vehicle, in a counterbalanced fashion, with one week separating treatments. Nicotine decreased both horizontal and vertical activity in all mice, regardless of prenatal anatoxin-a treatment. Thus, no enduring effects of prenatal anatoxin-a were obtained in adult mice following nicotine challenge.

Responses of the rat foetus to maternal injections of adrenaline and vasopressin.

1. Injection of 0.5-2.0 units of vasopressin or 25-100 mug of adrenaline into the peritoneal cavity of pregnant rats produced a transient slowing of the foetal heart. The bradycardia could be induced in foetuses after 15-21 days of gestation. Foetal heart rates dropped from normal values of 140-180 beats/min, often to less than 20 beats/minute. The period of bradycardia was dose dependent and ranged from 30 to 65 minutes.2. Maternal injection of the hormones produced a fall in foetal blood pressure from an average of 54, often to less than 20 mm of water, in 17-day foetuses. Direct injection of the hormones into the pericardial sac of the foetuses had the opposite effect and pressures rose an average of 15 mm of water 1 min after the injection.3. During the period of bradycardia, the potassium concentrations in foetal serum rose from an average value of 8.9 mequiv/1. to an average of 17.3 mequiv/litre. Concentrations of serum sodium fell from 126.2 to 121.4 mequiv/1. during the bradycardia. No changes were detected in the concentrations of either calcium or chloride. Foetal P(O2) levels fell from 25 to 15, P(CO2) rose from 61 to 89 or more, and pH fell from 7.19 to 6.86 during the bradycardia.4. Maternal death and uterine clamping caused foetal bradycardia and a rise in foetal serum potassium to an average of 20.2 mequiv/litre.5. It is concluded that interruption of normal uterine blood flow by vasoconstruction (adrenaline or vasopressin) or direct blockage (uterine clamping) results in a transient hypoxia, bradycardia, and serum ion changes in foetuses.

Disposition of 14C and/or 74As-cacodylic acid in rats after intravenous, intratracheal, or peroral administration.

The distribution, excretion, and possible metabolism of (14)C- and/or (74)As-cacodylic acid, an organoarsenical herbicide, was studied in rats following a single intravenous injection, intratracheal instillation or oral gavage. Male Sherman rats were dosed at levels ranging from 200 mg/kg to 120 mug/kg. The extent and rate of lung absorption was greater than gastrointestinal absorption. Concentrations in the liver and whole blood were higher after peroral dosing than intravenous administration. Levels observed in plasma and other tissues were similar after all three routes following the absorptive phase. The percent dose found in the whole blood, red blood cells, and plasma was similar for all doses given by these routes. Less than 0.1(1/2) of the administered dose was recovered as (14)CO(2) by any route at 24 hr after administration. Twenty-four hours after intravenous, intratracheal, and peroral administration, 71, 60, and 25%, respectively, was excreted in the urine. After intravenous administration of 200 mg/kg, sufficient (14)C-cacodylic acid was recovered in bile to account for the small amount excreted in the feces. Cacodylic acid is probably not metabolized to inorganic arsenic since the disposition of (14)C and (74)As-cacodylic acid were identical.Kinetic analyses of the plasma curve for (14)C-cacodylic acid (high dose) yielded three half-times; 0.014, 0.214 and 3.42 hr with an apparent volume of distribution of 15.3 ml. Highest initial concentrations were found in the whole blood, muscle, kidney, liver and lung. Levels in all tissues decreased rapidly, but remained high in whole blood. The red blood cells were found to be the major site of body burden of cacodylic acid.

Overview of a workshop on quantitative models for developmental toxicity risk assessment.

A workshop was held to discuss potential advancements to improve the precision of risk estimates for developmental toxicity. This paper presents an overview of the discussions at the workshop, focusing on the risk assessment process and science policy considerations important in the use of quantitative models. Some of the pertinent biological considerations are reviewed, particularly those related to the repair capacity of the developing organism and how this affects the concept of a threshold for developmental toxicity effects, as well as the maternal and litter influences on developmental toxicity outcomes. Finally, the current status of use of quantitative approaches is described, possible short-term approaches are discussed, and future research needs in this area are outlined.

Restraint-induced stress in pregnant mice--degree of immobilization affects maternal indices of stress and developmental outcome in offspring.

Maternal stress on gestational day 8 (GD8) in the CD-1 mouse can induce a syndrome of fetal anomalies, including encephalocele, supernumerary ribs, fused ribs and vertebral anomalies. Two forms of restraint were compared for their ability to induce these defects. The two types of restraint differed in the degree of mobility afforded the dam during confinement, either total restraint in a supine position or a less confining restraint in which restrained dams could move from a supine to a non-supine position, but could not turn from front to back. Dams were exposed to either form of restraint for 12 h on GD8 and their near-term fetuses examined for external and skeletal abnormalities. As both types of restraint precluded normal eating and drinking, an additional control group deprived of food/water was included for evaluation. Cohorts of dams were restrained for 3, 6 or 12 h on GD8 and end points commonly used to gauge the degree of stress evaluated. These included serum corticosterone level and the weight of body, spleen and thymus. Stress-induced analgesia, as measured by the tail-flick procedure, was monitored in these same dams as an additional non-invasive measure of stress. Both types of restraint induced greater and longer-lasting weight loss than food/water deprivation. Both also produced more fetal anomalies than observed in the offspring of caged controls or food/water deprived dams. Both forms of restraint equally elevated serum corticosterone levels above the increase exhibited by the food/water deprived dams. The most pronounced difference between the two types of restraint concerned the degree of analgesia. The type limiting mobility the most, caused much greater analgesia after 6 and 12 h of restraint although the dams subjected to the other form of restraint were significantly more analgesic than the food/water deprived dams by 12 h. Dams restrained in the supine position exhibited slightly greater weight loss, more analgesia and produced significantly more offspring with anomalies.

A review of the literature on potential reproductive and developmental toxicity of electric and magnetic fields.

The potential of electric and magnetic fields to adversely affect the health of the human population is an issue which continues to receive a great deal of attention in both public and scientific forums. One of the critical issues is the possibility that such fields may adversely affect the reproductive process. Numerous studies investigating the potential of electric and/or magnetic fields to alter reproduction in vertebrates have been conducted. These studies have, in many instances, yielded seemingly contradictory results. A number of epidemiological studies have been conducted as well. This review of the literature examines relevant studies and attempts to draw biologically rational conclusions from them. The studies are ordered in broad categories based upon both classification of the species studied (i.e. submammalian, mammalian exclusive of man and human) and the agent used (i.e. extremely low frequency electric, very low frequency electric, and magnetic fields). From our review we conclude that laboratory experimental and epidemiological results to date have not yielded conclusive data to support the contention that such fields induce adverse reproductive effects under the test or environmental conditions studied. Additional studies may, however, be warranted to clarify some of the experimental results obtained.

An overview of maternal toxicity and prenatal development: considerations for developmental toxicity hazard assessments.

The objective of testing xenobiotics for potential developmental toxicity is to extrapolate laboratory animal information to the human species, thereby deriving biologically rational regulatory policies. One of the problems that significantly contributes to the difficulty of this task is the possibility that general effects on the maternal organism could affect the developing conceptus. Published data have indicated that factors intrinsic to the maternal organism affect developmental outcome. This overview examines factors which may bear directly or indirectly upon developmental outcome, with emphasis on those of greatest relevance to the hazard assessment process. Standard teratology testing protocols often call for dose levels that induce overt maternal toxicity, and the developmental effects of this toxicity (both alone, and with concurrent embryo/fetal insult) continue to present regulators with considerable interpretive difficulties. In response to these problems there have been both research and literature review efforts dealing with the relationship of maternal and developmental toxicity. Relevant studies are reviewed here, and suggestions for avenues of future research are offered including the identification of any syndromes of developmental effects occurring at maternally toxic levels irrespective of the causative agent, and experimental approaches for the characterization of maternal toxicity.

Developmental effects of maternal stress in the CD-1 mouse induced by restraint on single days during the period of major organogenesis.

Maternal stress during gestation can produce significant fetal and/or postnatal effects, and can enhance the teratogenicity of other agents. We have previously shown that restraint stress on gestational day 8 in CD-1 mice produces significant increases in encephaloceles and supernumerary and fused ribs. In the present study we have examined the effects of stress induced by restraint on individual days during the period of major organogenesis (days 6-14). Weight loss and stress-induced analgesia as assessed by the tail-flick method were used to determine the degree of stress induced by a 12-h restraint period. Restrained animals lost significantly more weight and had longer tail-flick latencies than the concurrent food and water deprived controls on all gestational days. Significant increases in embryo/fetal mortality were also observed in the offspring of restrained animals. An increased incidence of supernumerary ribs was found in mice restrained on days 7 and 8. Since maternal toxicity induced by chemical teratogens may be accompanied by a general increase in maternal stress, our data suggest that such stress may be an etiological factor in teratology bioassays in which dose levels are sufficiently high to induce overt maternal toxicity.

Teratogenic effects of the demethylating agent 5-aza-2'-deoxycytidine in the Swiss Webster mouse.

5-Aza-2'-deoxycytidine (d-AZA) replaces cytidine in DNA thereby altering gene expression by passively removing methyl groups. This study determined the temporal patterns of morphological defects induced by d-AZA in mice. The dosages (0, 0.3, or 1.0 mg/kg) were administered by a single i.p. injection on gestational days (GD) 8, 9, 10, or 11. Mice were killed on GD 17 and fetal skeletons examined. The 1.0 mg/kg dose elicited characteristic defects for each treatment day: GD 8, supernumerary ribs, (significantly above background), fused vertebrae and ribs; GD 9, cleft palate and vertebral variations; GD 10, hind limb defects (especially phocomelia); GD 11, digital defects of fore and hindlimbs. The known demethylating ability of d-AZA coupled with the induction of longbone defects only in the hindlimbs suggests that d-AZA may act by disrupting specific hindlimb gene function through DNA hypomethylation.

Perinatal toxicity of endrin in rodents. III. Alterations of behavioral ontogeny.

The behavioral development of rats and hamsters was observed following perinatal exposure to endrin, a central nervous system teratogen in the hamster but not the rat [1,2]. In the hamster, prenatal exposure to endrin at 1.5 mg/kg/day on days 5-14 of gestation produced a persistent elevation in the locomotor activity. Offspring of treated hamsters ambulated 75% more than controls in the open field at 15 days and 45% more at 20 days of age. Long term observations of locomotor activity in the figure-8 mazes indicated that a significant elevation of this behavior was still present at 125 days of age. Non-locomotor behaviors of the Offspring (including sexual, rearing and running wheel behaviors) were unaffected. The dams repeatedly exposed daily to endrin at 0.75 or 1.5 mg/kg/day were markedly hypoactive using the same testing conditions in which the pups were hyperactive. This dosing regime was toxic to the dams in the 1.5 mg/kg/day dose group, killing more than half of them. In the second experiment, rats exposed perinatally to endrin at 0.15 or 0.30 mg/kg/day were 30% more active than control prior to weaning, but not as adults. These doses did not kill dams or affect the pup survival or growth. The similarity of the behavioral changes noted in the young of both species is suggestive of similar alteration of central nervous system function even though endrin produces gross morphological defects only in the hamster.

Perinatal toxicity of endrin in rodents. II. Fetotoxic effects of prenatal exposure in rats and mice.

The fetotoxic potential of endrin in the CD rat and CD-1 mouse was investigated. Endrin was administered as a solution in corn oil to groups of pregnant animals by gastric intubation at multiple dose levels throughout the period of organogenesis. The dams were sacrificed prior to term and the fetuses were examined for skeletal and visceral anomalies. In addition, maternal livers and fetuses from rats in each dose level were analyzed for endrin content. In the mouse, endrin caused maternal liver enlargement at a dose of 0.5 mg/kg/day and reduced maternal weight gain at a dose of 1.0 mg/kg/day. Fetal weight and skeletal and visceral maturity were adversely affected at a dose of 1.0 mg/kg/day, but no teratogenic effect or embryo lethality was evident even at a dose level that produced maternal lethality (1.5 mg/kg/day). In the rat, endrin markedly reduced maternal weight at doses above 0.150 mg/kg/day but produced no apparent effects on the fetus. The data suggest that species differences in sensitivity to endrin may in part be due to differences in metabolism. Although endrin levels in rat fetuses at a maximally tolerated dosage level resembled those previously reported for the hamster, relatively less 12-ketoendrin was present, paralleling the change in fetal sensitivity.

Perinatal toxicity of endrin in rodents. I. Fetotoxic effects of prenatal exposure in hamsters.

The potential of the insecticide endrin to induce fetal toxicity was determined in hamsters exposed to the compound on either day 8 or days 5--14 of gestation. Endrin was administered by oral gavage as a solution in corn oil. Doses used included 0.5--10.0 mg/kg on day 8 and 0.75 to 3.5 mg/kg/day on days 5--14. Exposure to a single dose of endrin resulted in significant incidences of fused ribs and meningoencephaloceles at levels of 5 mg/kg or greater. No significant effects were noted in either maternal mortality and weight gain or in fetal mortality or weight gain. The administration of multiple doses of endrin resulted in few fetal defects, although a significant dose-related increase in fetal mortality and decrease in fetal weight was seen. Significant maternal lethality and weight reductions were noted at doses of 1.5 mg/kg/day or greater. At sacrifice, maternal liver and fetal tissues were collected and subsequently analyzed for endrin and a major metabolite, 12-ketoendrin. Endrin was found to cross the placenta and 20 ppb were found in fetuses from litters exposed to 2.5 mg/kg/day. Maternal livers from this dose group contained an average of 2500 pbb of endrin.

Postnatal evaluation of prenatal exposure to p-xylene in the rat.

Pregnant Sprague-Dawley rats were exposed to either 3500 or 7000 mg/m3 p-xylene from days 7-16 of gestation. Dams were allowed to give birth, and litters were counted, weighed, and observed for external malformations on postnatal days (PD) 1 and 3. Litters were normalized to 8 pups (4 males and 4 females +/- 1) on PD4. On PD21 animals were weaned and littermates housed by sex. Body weights were recorded weekly until weaning and once every 2 weeks thereafter. Central nervous system (CNS) development was evaluated by acoustic startle response on PD13, 17, 21, and 63 as well as figure-8 maze activity on PD22 and 65. Maternal weight gain during the treatment period was significantly less in the high-dose group. No effects were seen on litter size or weight at birth or on PD3. There were no effects of xylene exposure on growth rate. There were no treatment-related effects on acoustic startle response or figure-8 maze activity. Thus, p-xylene as administered in this study does not appear to be a selective developmental toxicant in the rat.