The effects of castration on the extensor digitorum longus, soleus and levator ani muscles of the adult male rat were examined. After castration, no change was observed in the number of sites or the binding affinity of [125I]-monoiodo-alpha-bungarotoxin to the nicotinic cholinergic receptor population of the slow fiber muscle (soleus). The two fast fiber muscles (extensor and levator) showed no change in binding affinity but a significant change in the number of sites. In these two fast fiber muscles, the number of sites begins to increase within the first few days after castration, peaks at 14 days and begins to decline. The results presented in this report demonstrate three major concepts: 1) influences on skeletal muscle mediated by one receptor population can change the characteristics of another receptor population; 2) at least some androgen effects on skeletal muscle are fiber type specific; and 3) the development of extrajunctional receptors is not solely the function of the nerve-muscle relationship.