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The fifth and sixth lumbar segments of the rat spinal cord were found to contain a sexually dimorphic nucleus, the spinal nucleus of the bulbocavernosus (SNB). The SNB, which contains motoneurons innervating perineal striated muscles in normal male rats, is diminished or absent in normal females and in males with a genetic mutation rendering them insensitive to androgens. The presence of the nucleus is apparently not dependent on genetic sex, but on the action of androgens. The motoneurons of the adult male SNB accumulate hormone after systemic injections of radioactive testosterone or dihydrotestosterone, but not estradiol, and the SNB motoneurons accumulate more of the injected androgens than do other motoneurons in the same spinal segments. These results demonstrate a morphological sex difference in hormone-sensitive motoneurons that are probably involved in the sexually dimorphic …