The spinal nucleus of the bulbocavernosus (SNB) consists of motoneurons innervating striated perineal muscles in male rats. The adult number of SNB motoneurons can be increased or decreased by perinatal manipulations with androgen or anti-androgen. The present results with thymidine autoradiography demonstrate that SNB motoneurons undergo their final mitosis on the fourteenth day of gestation. Because testosterone production in male rats does not begin until after gestational day 14, androgens are unlikely to affect SNB neurogenesis in normal males. By extension, hormonal manipulations more than a week later, which affect the number of SNB cells, are probably mediated by alterations in the death or specification of cells, but not their proliferation.