Penile erection is a vascular phenomenon that results from smooth muscle relaxation, arterial dilation and venous restriction. The atherosclerosis of the penis that occurs with aging causes a decrease in penile oxygen tension. A reduction of smooth muscle cells has been demonstrated in relation with this change in oxygen tension. Changes in the ratio of penile collagen have also been observed and could explain the decrease in penile elasticity and compliance. Chronic ischemia is, therefore, associated with fibrosis but also with nitric oxide (NO)-cyclic guanosine monophosphate. The sensitivity of the alpha-adrenoceptors on the smooth muscle cells increases with aging. All those modifications can explain the prevalence of erectile dysfunction with aging. Low oxygen tension in prostanoid production may also play a role in the mechanism of ischemia-induced cavernosal fibrosis; however, intracavernous injections of prostaglandin E(1) do not seem to modify the intracavernous structures by reducing muscular atrophy. The effects of androgen on libido and sexual behavior are well established, but their role in the human erectile mechanism remains unclear. Several studies performed on animals have demonstrated impacts directly on both the physiological function and the trabecular structure of the corpora cavernosa in rats, dogs and rabbits. However, in humans, no study seems to demonstrate a role of testosterone on muscular atrophy or penile neurologic control. Testosterone treatment alters the human behavior but not penile physiologic processes. Further studies are necessary to explain the real role of testosterone not only on the peripheral mechanism of erection but also on the central control.