Initial rates of passive and carrier-mediated D-galactose and D-glucose uptake were measured in membrane vesicles derived from the maternal surface of the human placental syncytiotrophoblast. Passive diffusion, as measured by L-glucose uptake, was slightly and continuously temperature-sensitive over a range 0-40 degrees C (Q10 = 1.1). Below approx. 26 degrees C, passive diffusion measured by D-galactose uptake in the presence of the inhibitor, cytochalasin B, was quantitatively similar to L-glucose uptake. Above this temperature, however, cytochalasin B appeared not to be as effective an inhibitor of carrier-mediated uptake. The initial rates of D-galactose carrier-mediated transport, generated at low concentration (10 microM) were very temperature-sensitive and yielded a non-linear Arrhenius plot. An Arrhenius plot of Vmax, generated with higher concentrations, was linear. The linearity of the Vmax Arrhenius plot, in conjunction with the high cholesterol content of this membrane preparation, suggests that a membrane lipid phase transition is not responsible for the non-linearity of the low concentration Arrhenius plot. A discontinuous temperature sensitivity of the interaction between D-galactose and the hexose transport system, as reflected by a marked sensitivity in Km, appears responsible for the non-linearity in this Arrhenius plot.