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Numerous drugs and endogenous compounds are efficiently excreted from the renal proximal tubule via carrier-mediated pathways. Transepithelial excretion of organic anions occurs via their accumulative transport from the blood into the proximal tubule cells across the basolateral membrane and subsequent secretion into the urine through the apical membrane. Here we report on the isolation of a novel complementary DNA from rat kidney that encodes a 551-amino acid residue protein (OAT1) with 12 putative membrane-spanning domains. When expressed in Xenopus laevis oocytes, OAT1 mediated sodium-independent para-aminohippurate (PAH) uptake (Km = 14.3 +/- 2.9 microM). The uptake rate of PAH was increased by the outwardly directed dicarboxylate gradient, consisting with the idea that OAT1 is an organic anion/dicarboxylate exchanger. OAT1 displayed remarkably wide substrate selectivity, covering endogenous substrates such as cyclic nucleotides, a prostaglandin and uric acid, and a variety of drugs with different structures (e.g. antibiotics, a nonsteroidal anti-inflammatory drug, diuretics, an antineoplastic drug, and a uricosuric drug). The Northern blot analysis and in situ hybridization revealed that OAT1 is exclusively expressed in the particular segment of the proximal tubule in the kidney. These data suggest that OAT1 is a multispecific organic anion transporter at the basolateral membrane of the proximal tubule. Isolation of OAT1 will facilitate elucidation of the molecular basis of drug kinetics and the development of new drugs lacking unwanted side effects.