An improved understanding of the physiology of penile erection has resulted from recent evidence that implicates nitric oxide as the principal mediator of erectile function. Previously, the neuroanatomy of erection in man was established with descriptions of the autonomic innervation of the pelvic organs and external genitalia. The basis upon which novel physiological concepts of erection relate to earlier neuroanatomical principles remains to be determined. In the present study these relationships were explored with nitric oxide synthase immunohistochemistry and reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemistry of select pelvic tissue specimens obtained from 4 men (3 at radical prostatectomy and 1 at autopsy). Nitric oxide synthase, the enzyme that catalyzes nitric oxide production, was identified in discrete neuronal locations, including the pelvic plexus, cavernous nerves and their terminal endings within the corporeal erectile tissue, branches of the dorsal penile nerves and nerve plexuses in the adventitia of the deep cavernous arteries. This distribution of nitric oxide synthase-containing nerves suggests that nitric oxide neuronally modulates local vascular smooth musculature of the penis. On this basis, nitric oxide is identified as a neuronal mediator of penile erection in man.