Androgens play a vital role in erectile function and are known to have a neuroprotective role in the nervous system. This study investigated, in a rat model, the effects of testosterone deprivation and replacement on the morphology of the dorsal nerve of the rat penis at the light microscopy level. Two weeks after castration, male rats were infused with vehicle alone or 44 mu g of testosterone for 2 weeks. Age-matched, sham-operated control animals were used for comparisons. Penile tissue samples were removed for histological analyses. The following parameters were assessed: (1) total myelin sheath thickness; (2) density of nerve fibers; and (3) axon cross-sectional area per nerve fiber. Castration resulted in a significant increase in axon cross-sectional area compared to that of the control and testosterone-treated animals (6.97+/-0.59 mu m(2) per fiber in control animals to 14.32+/-0.44 mu m(2) per fiber in castrated animals). Qualitatively, there were signs of nerve degeneration, particularly myelin sheath degeneration, in all sample groups. We did not observe statistically significant changes in myelin sheath thickness. There was a trend of reduced nerve density. Nerve degeneration was not quantified since this study was performed at the light microscopic level. This study suggests that testosterone has a neuroprotective role in the nerve fibers of the dorsal nerve and testosterone deficiency may lead to different forms of nerve degeneration resulting in anatomic alterations, thus contributing to erectile dysfunction.