The present study deals with morphological changes during erection both in the penile deep artery supplying the corpus cavernosum and in veins draining it. These vessels were shown to maintain wide lumina in the flaccid state. In the erect state induced by injection of papaverine, the postcavernous venules were compressed between the cavernous sinuses and the tunica albuginea. Simultaneously, the penile deep artery and the penetrating vein were strangulated by stretched collagen bundles in the inner layer of the tunica albuginea. Quite noteworthy was a wedge-shaped connective tissue mass between the penetrating vein and the postcavernous venules converging to it, and plugged, in the erect state, towards the penetrating vein to effectively stem the venous outflow. When the rigid state was induced by papaverine injection and following intracavernous injection of a fixative by high pressure, the arterial and venous obliteration mentioned above was so intense that the vessels almost lost their lumina, resulting in the cavernous sinuses becoming isolated from the systemic circulation.