The present studies were designed to determine the role of reactive vascular smooth muscle in the regulation of blood flow into and out of the cavernous sinuses during penile erection in castrated and testosterone-treated animals. While the mean arterial pressure and intracavernosal pressure were continuously monitored, vasoactive drugs were injected into the aorta or into the cavernous sinuses during erection. The results show that both a NO releasing vasodilatory drug and an alpha adrenergic agonist significantly affected both mean arterial pressure and intracavernosal pressure when injected into the aorta. However, when these same drugs were injected into the cavernous sinuses, neither drug exerted a significant influence on the erectile response. Based on these studies, we conclude that the flow of blood into the cavernous sinuses during erection is regulated by reactive vascular smooth muscle but the outflow is not under the regulation of reactive vascular smooth muscle. Furthermore, the relaxation of the smooth muscle which controls the flow of blood into the cavernous sinuses during erection may be under partial androgenic control.