RESUMEN
1. Experiments involving polyol and sugar transfer across the guineapig placenta have been described.2. Regarding the former, sorbitol, dulcitol and mannitol are all transferred at approximately the same rate and more slowly than meso-inositol. Erythritol is not transferred.3. Concerning the hexose transfers, glucose and galactose are transferred at approximately the same rate and more rapidly than fructose.4. With regard to glucose transfer, there appears to be a mechanism with the following properties: (a) It tends towards saturation under certain conditions. (b) It favours saturation rather than diffusion kinetics for forward flows. (c) It exhibits competition with fructose and with galactose. Glucose/galactose competition has been used to demonstrate uphill transport by counterflow. (d) Competitive inhibition by phlorrhizin or phloretin could not be demonstrated.