A clinical drug-drug interaction between famotidine (a H 2 receptor antagonist) and probenecid has not been reproduced in rats. The present study hypothesized that the species-dependent probenecid sensitivity is due to a species difference in the contribution of renal organic anion and cation transporters. The transport activities of the H 2 receptor antagonists (cimetidine, famotidine, and ranitidine) by rat and human basolateral organic anion and cation transporters [human organic anion transporter (hOAT) 1, hOAT2, r/hOAT3, rat organic cation transporter (rOct) 1, and r/hOCT2] were compared using their cDNA transfectants. The transport activities (V max /K m ) of famotidine (K m , 345 M) by rOat3 were 8-and 15-fold lower than those of cimetidine (K m , 91 M) and ranitidine (K m , 155 M), respectively, whereas the activity by hOAT3 (K m , 124 M) was 3-fold lower than that of cimetidine (K m , 149 M) but similar to that of ranitidine (K m , 234 M). Comparison of the relative transport activity with regard to that of cimetidine suggests that famotidine was more efficiently transported by hOAT3 than rOat3, and vice versa, for ranitidine. Only ranitidine was efficiently transported by hOAT2 (K m , 396 M). rOct1 accepts all of the H 2 receptor antagonists with a similar activity, whereas the transport activities of ranitidine and famotidine (K m , 61/56 M) by r/hOCT2 were markedly lower than that of cimetidine (K m , 69/73 M). Probenecid was a potent inhibitor of r/OAT3 (K i , 2.6 -5.8 M), whereas it did not interact with OCTs. These results suggest that, in addition to the absence of OCT1 in human kidney, a species difference in the transport activity by hOAT3 and rOat3 accounts, at least in part, for the species difference in the drug-drug interaction between famotidine and probenecid.The kidney plays important roles in the detoxification of xenobiotics and endogenous wastes as well as maintaining the correct balance of ions and nutrients in the body. Urinary excretion is the major detoxification mechanism in the kidney, and this is governed by glomerular filtration, tubular secretion across the proximal tubules, and reabsorption. The renal uptake of organic anions and cations on the basolateral membrane of the proximal tubules has been characterized by multispecific organic anion and cation transporters (OAT/ SLC22 and OCT/SLC22), respectively (Lee and Kim, 2004;Wright and Dantzler, 2004;Shitara et al., 2005).Molecular cloning of basolateral transporters from different species allows examination of differences in their substrate specificities and transport activities, leading to a better understanding of the molecular mechanisms of species differences in drug disposition. For OCTs, the isoform expressed in the kidney differs between rodents and humans. Both Oct1 (Slc22a1) and Oct2 (Slc22a2) are involved in the renal uptake of organic cations on the basolateral membrane of the proximal tubules in rodents, whereas OCT2 is the predominant isoform in the human kidney (Koepsell, 2004;Lee and Kim, 2004;Wright a...