The renal secretion of organic anions across the proximal tubules is achieved by a coordination of uptake and efflux transporters. This study reports the expression, localization, and functional properties of mouse renal-specific transporter (RST). Mouse RST mRNA is predominantly expressed in the kidney and localized on the brush border membrane of mouse kidney proximal tubules. Mouse RST-expressing HEK293 cells exhibited saturable uptake of p-aminohippurate (Km ∼234 μM), which was increased by an increase in K+ concentration or in the presence of Ba2+ and ouabain and decreased by diethylpyrocarbonate, a histidine modifier. An increase in K+ concentration enhanced the uptake of benzylpenicillin, 2,4-dichlorophenoxyacetate, and dehydroepiandrosterone sulfate, suggesting polyspecific substrate specificity of mouse RST. Vectorial transport of 2,4-dichlorophenoxyacetate was observed in the basal-to-apical direction in rat organic anion transporter 3-expressing LLC-PK1 cells (rOat3-LLC); however, coexpression of mouse RST in rOat3-LLC caused a 1.3-fold increase in the basal-to-apical transport. In addition, the basal-to-apical transport of benzylpenicillin and urate was 3- and 2.5-fold greater than that in the opposite direction in the double-transfected cells, respectively, whereas their transepithelial transport in vector- or rOat3-LLC was symmetrical. Furthermore, the basal-to-apical transport of benzylpenicillin was saturable and reduced by increasing extracellular K+ concentration and ouabain. These results suggest that mouse RST mediates the efflux of organic anions including urate and works as exit for organic anions in the proximal tubules. In addition to the kidney, mouse RST was detected in the brain capillaries and the choroid plexus, and it may also play a role in efflux transport of organic anions across the barriers of the central nervous system.