P-glycoprotein (P-gp), the MDR1 multidrug transporter, is known to be expressed in several human organs and tissues, including the apical membrane of the renal proximal tubular cells. It has been reported that human immunodeficiency virus 1 (HIV-1) can trigger the expression of P-gp in cultured cells (i.e., H9, a T-lymphocyte cell line, and U937, a monocyte cell line), which may render the cells resistant to antiretrovirals. Since multiple membrane transport systems (i.e., organic cation, organic anion, and nucleoside systems) can be involved in the renal tubular transport of dideoxynucleoside analog drugs (DADs) (i.e., zidovudine and zalcitabine), we have questioned if P-gp is involved in the renal transport of DADs. Chinese hamster ovary colchicine-resistant cells (CH(R)C5), a cell line that is well known to highly express P-gp, and continuous renal epithelial cell lines (LLC-PK1 and OK), which have also been shown to express P-gp, were used. The accumulation of [3H]vinblastine (20 nM), an established P-gp substrate, by the monolayer cells was significantly enhanced in the presence of two P-gp inhibitors (i.e., verapamil and cyclosporin A) and nucleoside transport inhibitors (i.e., dipyridamole and dilazep). In contrast, DADs (i.e., zidovudine, lamivudine, didanosine, and zalcitabine) did not significantly affect vinblastine accumulation by these cell lines. These data suggest that P-gp does not play a significant role in the renal tubular transport of DADs. Dipyridamole and dilazep, two nucleoside membrane transport inhibitors, appear to be P-gp inhibitors.