We previously reported that a motor nucleus of the rat spinal cord, the spinal nucleus of the bulbocavernosus (SNB), has more and larger neurons in males than females. The SNB is significantly masculinized in adult female rats receiving 1 mg of testosterone propionate (TP) on the second day of life, compared to that of their oil treated sisters. This masculinization consists of more neurons in the region of the SNB. These results are consistent with the hypothesis that the action of androgens during the early perinatal period is responsible for the sexually dimorphic development of the SNB. Neonatal TP caused no detected masculinization, i.e. enlargement of the size of individual neurons in this region in females. Male rats given TP on day 2 had significantly smaller testes and smaller SNB somas in adulthood than oil treated males. In female rats given 100 micrograms of estradiol benzoate (EB) on day 2, the SNB was not masculinized in either number of size when observed in adulthood. The lack of a masculinizing effect of EB suggests that the aromatization of testosterone to estradiol is not sufficient for this dimorphic development.