Endolymph in membranous labyrinth is a K+-rich and Na+-poor fluid, and perilymph is conversely Na+-rich and K+-poor. Electrolyte transport between endolymph and perilymph is important for regulation of volume and osmotic pressure of the labyrinth. The epithelial sodium channel (ENaC) is a good candidate protein for Na+ transport in the tight epithelia, which has been well demonstrated in other tissues such as kidney, colon and lung. The purpose of the present study was to investigate the cellular localization of ENaC subunits in the rat inner ear immunohistochemically with the specific polyclonal rabbit antibodies against the rat α-, β- and γ-ENaC. All three subunits of ENaC were extensively labeled in the cochlea including the stria vascularis, spiral ligament, organ of Corti, spiral limbus, Reissner's membrane and spiral ganglion, and in the vestibule including the sensory epithelia and stroma cells of the macula utriculi, macula sacculi and ampullary crest. In conclusion, our results suggest that functional ENaC in the labyrinth may work in concert with other Na+ and K+ transport molecules to regulate endolymph and to maintain homeostasis in the inner ear.