The spinal nucleus of the bulbocavernosus (SNB) innervates striated muscles, the bulbocavernosus and levator ani (BC/LA), which control penile reflexes. Castration results in shrinkage in the size of SNB somata and dendrites, as well as BC/LA muscle mass. However, there is no information about how quickly these regressive changes occur compared to the rapid effects of castration upon penile reflexes, which are greatly diminished a few days after surgery. Therefore we examined the time course of change in the size of SNB somata after castration of adult male rats. Males were sacrificed 2, 14, or 28 days after either castration or sham surgery and somata were measured in the SNB and in a control population of motoneurons, the retrodorsolateral nucleus (RDLN). BC/LA weight was reduced in castrates compared to intact males 14 and 28 days post surgery, but SNB somata were significantly smaller in castrates only at 28 days after surgery. As has been previously observed, castration did not affect soma size in the RDLN. These data indicate that SNB somata respond more slowly after castration than BC/LA mass or penile reflexes, suggesting that the size of SNB somata cannot account for the loss of penile reflexes. Androgenic effects on SNB somata may contribute to aspects of reproductive behavior that are not apparent in penile reflexes tested ex copula.