In adult male rats, motoneurons of the spinal nucleus of the bulbocavernosus (SNB) have been shown to retract and reextend their dendritic branches in response to systemic androgen deprivation and readministration. Furthermore, other studies have suggested that the dendritic complexity of neurons can be regulated by their targets. To assess whether androgens might act upon the target muscles to mediate changes in SNB dendrites, adult male rats were castrated and implanted with a small capsule filled with testosterone (T) next to the bulbocavernosus and levator ani muscle complex (BC/LA) on one side, while the muscles on the contralateral side were implanted with another capsule containing hydroxyflutamide (hFl), an anti-androgen. We have previously shown that after 30 d of this focused, lateralized androgen treatment the BC/LA complex is significantly larger on the T-treated side. We now report that the total dendritic lengths of SNB motoneurons innervating muscles given androgen blockade are reduced by 44% compared to SNB motoneurons innervating muscles given androgen stimulation. Dendrite lengths within three regions of the spinal cord were altered in a nonuniform manner: large changes occurred in the dorsal and contralateral dendritic fields while there was no difference in the ipsilateral dendritic field. These results suggest that BC/LA muscles, in response to androgen stimulation, produce a trophic substance which regulates the dendritic organization of SNB motoneurons in adulthood.