Organic cation transporters play an important role in the secretion of cationic drugs as well as endogenous cationic metabolites in the renal tubules. Immunoblotting showed the presence of organic cation transporter proteins, rOCT1 and rOCT2, in the rat kidney. By immunofluorescence microscopy, rOCT1 was shown to be concentrated in the proximal tubules in the renal cortex. rOCT2, on the other hand, was rich in the proximal tubules in the outer stripe of the outer medulla. Confocal microscopy revealed that both rOCT1 and rOCT2 were localized to the basolateral membranes of these tubule cells. These findings directly show that rOCT1 and rOCT2 are basolateral membrane proteins and are differentially distributed along the proximal tubules.