Osteopontin (OPN) is considered to be a non-collagenous bone matrix protein that is involved in the ossification process. However, OPN has recently been observed in ectopic sites such as the kidney and nervous tissue. In the present study, the expression of OPN mRNA was examined in the rat and mouse inner ear by nonradioisotopic in situ hybridization. Signals of OPN mRNA were observed in the marginal cells of the stria vascularis, spiral ganglion cells, vestibular sensory hair cells, and vestibular dark cells. OPN protein was detected only in the otoconia by immunohistochemistry. The presence of OPN mRNA in the cochlea and vestibular dark cells may indicate that OPN is involved in the regulation of ions in the inner ear fluid. Findings in the saccule and utricle suggest that OPN is one of the protein components of the rodent otoconia and that the vestibular sensory hair cells are involved in the production of otoconia.