RESUMEN
The insectivorous bat Myotis nigricans is widely distributed throughout the Neotropics, including Brazil, and has a reproductive biology that is affected by climate and food availability. To evaluate the reproductive capacity of this species, morphofunctional parameters of the testes were correlated with environmental variables and the body condition of individuals captured. After bats had been killed, their testes were removed, fixed in Karnovsky's fluid for 24h and embedded in resin for evaluation by light microscopy. The mean annual tubulosomatic index (0.58%) and the percentage of seminiferous tubules in the testes (88.96%) were the highest ever recorded for the Order Chiroptera. The percentage of Leydig cells and volume of the cytoplasm of Leydig cells were higher in the rainy than dry season (80.62±3.19% and 573.57±166.95µm, respectively; mean±s.d.). Conversely, the percentage of nuclei of the Leydig cells in the dry season (26.17±3.70%; mean±s.d.) and the total number of Leydig cells (6.38±1.84×109; mean±s.d.) were higher in the dry season. The results of the present study could help in future conservation of these bats because they provide a better understanding of the bats' reproductive strategies and how the species can adapt to changes.