Recent studies indicate that neurotrophin 3 (NT3) may be important for the maintenance and function of the adult inner ear, but the pattern of postnatal NT3 expression in this organ has not been characterized. We used a reporter mouse in which cells expressing NT3 also express β-galactosidase, allowing for their histochemical visualization, to determine the pattern of NT3 expression in cochlear and vestibular organs. We analyzed animals from birth (P0) to adult (P135). At P0, NT3 was strongly expressed in supporting cells and hair cells of all vestibular and cochlear sense organs, Reissner's membrane, saccular membrane, and the dark cells adjacent to canal organs. With increasing age, staining disappeared in most cell types but remained relatively high in inner hair cells (IHCs) and to a lesser extent in IHC supporting cells. In the cochlea, by P0 there is a longitudinal gradient (apex > base) that persists into adulthood. In vestibular maculae, staining gradients are: striolar > extrastriolar regions and supporting cells > hair cells. By P135, cochlear staining is restricted to IHCs and their supporting cells, with stronger expression in the apex than the base. By the same age, in the vestibular organs, NT3 expression is weak and restricted to saccular and utricular supporting cells. These results suggest that NT3 might play a long-term role in the maintenance and functioning of the adult auditory and vestibular systems and that supporting cells are the main source of this factor in the adult. J. Comp. Neurol. 501:30–37, 2007. © 2007 Wiley-Liss, Inc.