cDNAs for novel organic cation transporters, OCTNI and OCTN2, were cloned from human fetal liver and adult kidney, respectively, and their transport activities were investigated. OCTN1 encodes a 551-amino-acid protein. It is strongly expressed in kidney, placenta, and several other tissues and in human cancer cell lines. When expressed in HEK293 cells, OCTN1 exhibited saturable and pH-dependent [¹⁴C]tetraethylammonium (TEA) uptake with higher activity at neutral and alkaline pH than that at acidic pH. In addition, OCTN1-transfected HEK293 cells exhibited a larger TEA efflux from the cells at an acidic extracellular pH. When OCTN1 was expressed in Xenopus oocytes, it transported TEA in a pH dependent manner with an increased activity at higher medium pH as the same as that expressed in HEK293 cells, whereas membrane potential or sodium ions had no effect. OCTN1-mediated TEA uptake was inhibited by various organic cations and OCTN1 transports cationic compounds such as [³H]pyrilamine, [³H]quinidine and [³H]verapamil. Furthermore, L-[³H]camitine was transported in a sodium dependent manner by HEK293 cells expressed with OCTN1 or OCTN2. Accordingly, OCTN1 was functionally demonstrated to be a multispecific proton/organic cation antiporter which presumably functions at the renal apical and other tissue membranes and the common physiological role of OCTN1 and OCTN2 might be transport of carnitine in several tissues.