1. Plasma membranes isolated from rat liver according to a modification of the method of Neville, D.M. ((1960) J. Biophys. Biochem. Cytol. 8, 413-422) were used as model to test current hypotheses on the mode of glucose uptake into the liver cell. Glucose uptake studies were performed by a filtration technique using labeled glucose analogues. 2. D-glucose Uptake by rat liver plasma membranes is characterized by features of simple diffusion, i.e. linearity of uptake, lack of stereospecificity, and by facilitated diffusion, i.e. temperature dependence, counterflow phenomenon and inhibition by phloretin. These findings confirm earlier studies on liver slices and perfused liver. 3. Binding studies on sonicated membranes provide evidence for a specific binding site or protein for D-glucose at the plasma membrane by isolating Tris-soluble membrane proteins which reveal a higher binding capacity than the unsonicated membrane. 4. These findings are interpreted as showing the presence of a "carrier" mediated transport system for D-glucose superimposed by free diffusion due to artificial disruption of the plasma membranes.