The aim of our study was to investigate the effects of rabeprazole, a proton pump inhibitor, on MDR1 expressed on human colon carcinoma cell line, Caco-2, and MDR1-overexpressing human cervical carcinoma cell line, HeLa cells selected by exposure to 100 n M vinblastine (Hvr100-6 cells). Inhibitory effects of rabeprazole on MDR1-mediated transport of Rhodamine123 were examined in these cells. A thousand micro molar rabeprazole increased Rhodamine 123 uptakes in Caco-2 and Hvr100-6 cells by 68% and 185%, respectively. No significant effects of rabeprazole were observed at the concentration of 1—100 μ M . Since rabeprazole did not show any effects on Rhodamine 123 transport via MDR1 at the plasma levels (approximately 1 μ M ), it was considered that the drug interaction with MDR1 substrates would be minimal even though the interaction occurred in the patients with rabeprazole treatment.