Absorption and reabsorption of amino acids are important functions of the small intestine and renal proximal tubule. A proton coupled amino acid transporter was identified functionally in the rabbit renal proximal tubule (1980s) and the human intestinal Caco-2 cell line (1990s). This transporter has now been identified at the molecular level from rat, mouse, human and rabbit and named PAT1 (for Proton coupled Amino acid Transporter 1). Immunocytochemical studies localise PAT1-like immunoreactivity solely to the apical membrane of Caco-2 cells and human and rat small intestinal enterocytes. Functional studies demonstrate that PAT1 is equivalent to a transporter identified in the rat small intestine in the 1960s which was named the imino acid carrier. The physiological role of PAT1 is to absorb/reabsorb a range of small, unbranched, dipolar amino acids, imino acids, beta amino acids, and orally-administered compounds used in the treatment of schizophrenia and epilepsy. PAT1 is the first member of solute carrier family 36 (SLC36A1) which consists of 4 related transporter-like sequences. PAT2 (SLC36A2) is also a proton coupled amino acid transporter. SLC36A3 and SLC36A4 are orphan transporters.