Recent evidence implicates NO (Nitric oxide) as the principal mediator in an erection. To investigate the role of NO in the human erectile function, we studied the distribution pattern of nitroxergic fibers in the corpus cavernosum specimens obtained from 38 men using nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry. Diffusely scattered delicate nerve fibers showing blue color reaction after NADPH-d histochemical staining were observed in normal control specimens from potent men. The neurogenic impotence group showed a statistically-significant decrease in the number of positive fibers compared to the normal control group. The number of positive fibers in the non-neurogenic impotence group was decreased compared to the normal control group but was statistically insignificant. With nitric oxide synthase (NOS) immunohistochemical stain, immunoreactive nerve bundles were easily seen in normal control specimens from potent men. NOS immunoreactive nerve bundles were contained within the corpus cavernosa which stained with NADPH-d reaction. Our results suggest that nitric oxide, a potent smooth muscle relaxing neurotransmitter in the autonomic nervous system, plays a physiologic role in erectile function and NADPH-d enzyme histochemical staining on the biopsied corpus cavernosum may be used as an important diagnostic method in the evaluation of neurogenic impotence.