The G protein of vesicular stomatitis virus was implanted in the apical plasma membrane of Madin-Darby canine kidney cells by low pH-dependent fusion of the viral envelope with the cellular membrane. The amount of fusion as determined by removal of unfused virions, either by tryptic digestion or by EDTA treatment at 0°C, was 22-24% of the cell-bound virus radioactivity. Upon incubation of cells after implantation, the amount of G protein as detected by immunofluorescence diminished on the apical membrane and appeared within 30 min on the basolateral membrane. At the same time some G protein fluorescence was also seen in intracellular vacuoles. The observations by immunofluorescence were confirmed and extended by electron microscopy. Using immunoperoxidase localization, G protein was seen to move into irregularly shaped vacuoles (endosomes) and multivesicular bodies and to appear on the basolateral plasma membrane. These results suggest that the apical and basolateral domains of Madin-Darby canine kidney cells are connected by an intracellular route.In most cells the plasma membrane is continuously endocytosed (36) and the loss of surface membrane must be balanced by retrieval of membrane from inside the cell. Membrane recycling poses a special problem in epithelial cells because the plasma membrane is polarized; it is divided into apical and basolateral domains with distinct protein and lipid compositions (35). If the plasma membrane is continuously recycling, an important question is how the apical and basolateral domains preserve their unique compositions. If the apical and the basolateral recycling routes are separated, randomization of the cell surface would of course be prevented. If, however, they connect at some point in the cell, continuous sorting of apical and basolateral proteins would have to take place.In an attempt to map the membrane traffic routes to and from the cell surface in epithelial cells, we are using the MadinDarby canine kidney (MDCK) cell line as our experimental system (7, 16). These cells display both structural and functional polarity when grown in culture (24, 32). The microvillar