This report describes the structure, function, and tissue distribution pattern of rat OCTN1 (novel organic cation transporter 1). The rat OCTN1 cDNA was isolated from a rat placental cDNA library. The cDNA is 2258 bp long and codes for a protein of 553 amino acids. Its amino acid sequence bears high homology to human OCTN1 (85% identity) and rat OCTN2 (74% identity). When expressed heterologously in mammalian cells, rat OCTN1 mediates Na(+)-independent and pH-dependent transport of the prototypical organic cation tetraethylammonium. The transporter interacts with a variety of structurally diverse organic cations such as desipramine, dimethylamiloride, cimetidine, procainamide, and verapamil. Carnitine, a zwitterion, interacts with rat OCTN1 with a very low affinity. However, the transport of carnitine via rat OCTN1 is not evident in the presence or absence of Na(+). We conclude that rat OCTN1 is a multispecific organic cation transporter. OCTN1-specific mRNA transcripts are present in a wide variety of tissues in the rat, principally in the liver, intestine, kidney, brain, heart and placenta. In situ hybridization shows the distribution pattern of the transcripts in the brain (cerebellum, hippocampus and cortex), kidney (cortex and medulla with relatively more abundance in the cortical-medullary junction), heart (myocardium and valves) and placenta (labyrinthine zone).