Sex differences and differences related to genotype have been observed in the nervous system. These observations provide the opportunity to relate differences in neural structure to functional differences. The spinal nucleus of the bulbocavernosus (SNB) was examined in castrated and gonadally intact male house mice from the following 3 genotypes: C57B1/6J, DBA/2J, and the B6D2F1 (F1) hybrid (resulting from a cross between a C57B1/6J female and a DBA/2J male). The number and size of SNB neurons were determined from thionin-stained thick sections from spinal cords of these mice. The gonadally intact DBA males had significantly fewer SNB motoneurons than either the C57 or F1 males, but no strain differences were observed for the size of SNB cells. Castration of adults significantly reduced SNB neuronal number, but not somatic area, in the C57s (the maternal strain) and reduced somatic area, but not neuronal number, in the DBAs (the paternal strain). Both characteristics of the SNB were reduced by castration in the F1 hybrid. Thus, the size and number of SNB neurons appear to be inherited independently and the influence of gonadal hormones on these traits varies with genotype.