The foot muscle, flexor digitorum brevis, is innervated by motoneurons in the retrodorsolateral nucleus of the lumbar spinal cord in rats. We found this muscle to be sexually dimorphic, but insensitive to the anabolic or catabolic effects of androgen manipulation in adulthood: the flexor digitorum brevis was larger in adult male rats than in females, with no decrease in mass after castration in males nor any increase in size after androgen treatment of ovariectomized females. The cross-sectional area of motoneurons innervating this muscle was also sexually dimorphic, i.e., the motoneurons were larger in males. In contrast to the absence of an androgen effect on target muscle size, however, cross-sectional area of motoneurons decreased in adult males as a result of castration, and increased in adult females after androgen treatment. The dissociation of androgen effects on muscle mass and motoneuron size suggests the possibility of steroid effects upon motoneurons independent of effects upon target musculature.