MATE1 was the first mammalian example of the multidrug and toxin extrusion (MATE) protein family to be identified. Human MATE1 (hMATE1) is predominantly expressed and localized to the luminal membranes of the urinary tubules and bile canaliculi and mediates H+-coupled electroneutral excretion of toxic organic cations (OCs) into urine and bile (Otsuka M, Matsumoto T, Morimoto R, Arioka S, Omote H, and Moriyama Y. Proc Natl Acad Sci USA 102: 17923–17928, 2005). mMATE1, a mouse MATE ortholog, is also predominantly expressed in kidney and liver, although its transport properties are not yet characterized. In the present study, we investigated the transport properties and localization of mMATE1. Upon expression of this protein in HEK-293 cells, mMATE1 mediated electroneutral H+/tetraethylammonium exchange and showed a substrate specificity similar to that of hMATE1. Immunological techniques with specific antibodies against mMATE1 combined with RT-PCR revealed that mMATE1 is also expressed in various cells, including brain glia-like cells and capillaries, pancreatic duct cells, urinary bladder epithelium, adrenal gland cortex, α cells of the islets of Langerhans, Leydig cells, and vitamin A-storing Ito cells. These results indicate that mMATE1 is a polyspecific H+/OC exchanger. The unexpectedly wide distribution of mMATE1 suggests involvement of this transporter protein in diverse biological functions other than excretion of OCs from the body.