Androgen activity in the central nervous system, as in other tissues, is mediated by the androgen receptor. We performed the precise localization of the androgen receptor in spinal cord of male and female adult rats by immunohistochemistry using polyclonal antibodies. Light microscopy indicated immunoreactivity in the anterior horn with a strong staining in motoneurons, but staining was also observed in the posterior horn. Electron microscopy showed a predominant nuclear immunostaining. A weaker but significant immunoreactive androgen receptor was also noted in the perinuclear/ intracysternal position. Moreover, no differences were found between male and female rats. Immunoblotting demonstrated that the androgen receptor is expressed in both ventral and dorsal spinal cord, with an apparent molecular mass identical to that noted in other androgen-dependent tissues. The expression of androgen receptor in motoneurons corroborates the role of androgens in motoneuron growth, development and regeneration and underlies the possibility that androgen receptor abnormality leads to the motoneuron degeneration observed in X-linked spinal and bulbar muscular atrophy.