There is a sexually dimorphic motor nucleus in the fifth and sixth lumbar segments of the rat spinal cord, consisting of motoneurons innervating two striated perineal muscles, the levator ani and the bulbocavernosus. This nucleus, which is diminished or absent in female rats, has been named the spinal nucleus of the bulbocavernosus (SNB)3. We now report that the number of neurons in the SNB of either male or female rats is not altered by adult gonadectomy or treatment with testosterone propionate. However, the size of individual SNB neurons is increased in the presence of androgen in either sex. Genetically male rats with the testicular feminization mutation which results in reduced receptors have a markedly feminine SNB. These results support the hypothesis that the sexually dimorphic nature of the SNB depends on neither the adult hormonal state nor the presence of a Y chromosome, but on the interaction of androgens with their receptors early in development.