The histochemical and ultrastructural changes in the mature feline bulbocavernosus muscle (BCM) induced after 10 days of denervation or castration were compared. Control BCM, denervated BCM, and BCM from castrated cats reacted similarly for myosin ATPase (predominantly dark staining) and for succinic dehydrogenase (predominantly high). Staining for phosphorylase was decreased in the denervated BCM and BCM from castrated cats. Intrafiber lipid content was reduced after denervation or castration. The appearance of lysosome-like structures in BCM from both treatment groups was the most obvious ultrastructural alteration. Other ultrastructural changes were focal and infrequent.