Resorption of amino acids in kidney and intestine is mediated by transporters, which prefer groups of amino acids with similar physico-chemical properties. It is generally assumed that most neutral amino acids are transported across the apical membrane of epithelial cells by system B⁰. Here we have characterized a novel member of the Na⁺-dependent neurotransmitter transporter family (B⁰AT1) isolated from mouse kidney, which shows all properties of system B⁰. Flux experiments showed that the transporter is Na⁺-dependent, electrogenic, and actively transports most neutral amino acids but not anionic or cationic amino acids. Superfusion of mB⁰AT1-expressing oocytes with neutral amino acids generated inward currents, which were proportional to the fluxes observed with labeled amino acids. In situ hybridization showed strong expression in intestinal microvilli and in the proximal tubule of the kidney. Expression of mouse B⁰AT1 was restricted to kidney, intestine, and skin. It is generally assumed that mutations of the system B⁰ transporter underlie autosomal recessive Hartnup disorder. In support of this notion mB⁰AT1 is located on mouse chromosome 13 in a region syntenic to human chromosome 5p15, the locus of Hartnup disorder. Thus, the human homologue of this transporter is an excellent functional and positional candidate for Hartnup disorder.