In rats, androgens in adulthood regulate the morphology of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), including the size of their somata and the length of their dendrites. There are conflicting reports about whether androgens exert similar influences on SNB motoneurons in mice. We castrated or sham-operated C57BL6J mice at 90 days of age and, thirty days later, injected cholera toxin conjugated horseradish peroxidase into the bulbocavernosus muscle (to label SNB motoneurons) on one side, and into intrinsic foot muscles contralaterally (to label motoneurons of the retrodorsolateral nucleus (RDLN)). Castrated mice had significantly smaller SNB somas compared to sham-operated mice while there were no differences in soma size of RDLN motoneurons. Dendritic length in C57BL6J mice, estimated in 3-dimensions, also decreased significantly after adult castration. In rats, androgens act directly through androgen receptors (AR) in SNB motoneurons to control soma size and nearly all SNB motoneurons contain AR. Since SNB somata in C57BL6J mice shrank after adult castration, we used immunocytochemistry to characterize AR expression in SNB cells as well as motoneurons in the RDLN and dorsolateral nucleus (DLN). A pattern of labeling matched that seen previously in rats: the highest percentage of AR-immunoreactive motoneurons are in the SNB (98%), the lowest in the RDLN (25%) and an intermediate number in the DLN (78%). This pattern of AR labeling is consistent with the possibility that androgens also act directly on SNB motoneurons in mice to regulate soma size in mice.