The retinal pigment epithelium (RPE) is unique in that Na, K-ATPase is predominantly localized on its apical surface. We studied the distributions of Na, K-ATPase and glucose transporter GLUT1, insulin and transferrin receptors in developing rat RPE cells immunocytochemically. Na, K-ATPase, first detected in 17-day-old embryonic eyes, was already distributed predominantly on the apical surface. This reversed distribution of Na, K-ATPase was maintained throughout their life. Insulin receptor and transferrin receptor were distributed exclusively on the basolateral surface. By quantitative immunogold electron microscopic technique we found that glucose transporter GLUT1 is distributed almost equal in amount on both the apical and basolateral surfaces of RPE cells, thus presumably constructing an efficient pathway for glucose transport from the choriocapillaries to the neural retina through the blood-retinal barrier. These results suggest that in the RPE cells the intrinsic basolateral plasma membrane proteins are sorted out at least in three different ways.