To analyze the mechanism of penile erection and pathogenesis of impotence, pressures in the corpus cavernosum in anesthetized dogs were measured. Pelvic nerve stimulation produced pressor responses in a frequency-dependent manner. Intravenous injections of N^G-nitro-L-arginine, a nitric oxide (NO) synthase inhibitor, dose dependently attenuated the response, and the inhibition was reversed by intravenous injection of L-arginine but not of D-arginine. The response was also inhibited by N^G-nitro-L-arginine injected into the corpus cavernosum, the potency being ~10 times of that applied intravenously. The intracavernous injection of L-arginine restored the response. N^G,N^G-dimethylarginine, an endogenous NO synthase inhibitor, dose dependently attenuated the stimulation-induced response, which was restored by an intracavernous injection of L-arginine. An intravenous injection of hexamethonium abolished the pressor response to nerve stimulation, whereas phentolamine and atropine did not significantly alter the response. These findings suggest that an increase in intracavernous pressure caused by pelvic nerve stimulation in anesthetized dogs is mediated by NO liberated from postganglionic neurons that originate in the ganglion located in the vicinity of corpus cavernosum.