The goal of this research was to investigate movement of sugars across placental plasma membranes. Changes in vesicle volume produced by solute uptake were measured by light scattering. Analysis, performed by fitting of the light scattering data to exponentials, revealed that for certain sugars such as glucose, a rapid component and a second, slower transport process were present. Measurements in the presence of the glucose transport inhibitor phloretin, comparison with the transport of mannitol and analysis of the concentration dependence of the two transport components were used to demonstrate that these two processes are consistent with protein-mediated and lipid-diffusional transport of glucose. Calculation of glucose flux rates using the time constants which define these processes provided values similar to those determined by radioisotopic methods. Glucose, 2-deoxyglucose and galactose were transported both by carrier-mediated and diffusional processes, while mannitol, fructose, ribose and 2-deoxyribose were transported solely by the latter process and not by a protein carrier. The rate of glucose transport across the syncytiotrophoblast basal membrane was slightly greater than that across the microvillous membrane, in contrast to that predicted previously by immunoblotting. In addition, measurements of hexose transmembrane diffusion showed that microvillous and basal transport rates were similar and lower than previously determined. We conclude that this new technique represents a simple and rapid method for investigating sugar transport across placental membranes.