Previous studies have shown that intracavernosal blood flow increases during penile erection, but little is known about intracavernosal hemodynamics. Using a previously developed canine model of erection, we measured intracavernosal blood flow and oxygen tension at 2 sites within the corpus cavernosum: directly beneath the tunica albuginea and deep within the cavernous tissue. We chose to measure oxygen tension as an indicator of arterial blood flow. Penile erection was induced by pelvic nerve stimulation as well as by injection of papaverine and phentolamine. In the flaccid penis, blood flow measured directly under the tunica albuginea was significantly higher than deep intracavernosal blood flow. Subtunical oxygen tension in the flaccid penis was consistent with a largely arterial circulation. These observations provide physiological evidence of an important subtunical circulation that carries most of the intracavernosal blood flow when the penis is flaccid. With pelvic nerve stimulation, deep intracavernosal blood flow increased significantly followed by an increase in oxygen tension. Oxygen tension deep within the corpus cavernosum increased during penile erection from a level consistent with venous blood to a level consistent with arterial blood. Injection of papaverine and phentolamine caused a significant increase in intracavernosal pressure and a significant decrease in subtunical blood flow but did not cause statistically significant change in intracavernosal blood flow or oxygen tension. In contrast to nerve-induced erection, pharmacologically induced erection appears to depend more on intracavernosal shunting of blood than on increased total arterial blood flow to the penis. Hypogastric nerve stimulation during established erection caused detumescence by contracting cavernosal smooth muscle, reducing deep cavernosal blood flow and reestablishing blood flow through the subtunical space. Our observations suggest that the subtunical space contains an important circulation that may play a role in the hemodynamics of the flaccid, as well as the erect, penis.