Penile hypercoagulability during erection may predispose to aging vascular changes and, eventually, arteriogenic impotence. The relationship of penile blood thromboxane B2 and 6-keto-prostaglandin F1 alpha in both psychogenically and arteriogenically impotent patients after intracavernosal treatment with papaverine plus phentolamine or prostaglandin E1 was evaluated. No significant change in penile blood thromboxane B2 was observed with treatment of these vasoactive drugs. On the other hand, penile blood 6-keto-prostaglandin F1 alpha was significantly increased with the injection of 30 mg of papaverine plus 0.5 mg of phentolamine, and of 20 micrograms prostaglandin E1. Furthermore, the prostacyclin-to-thromboxane A2 ratio for the patient who received papaverine plus phentolamine was significantly lower than that for the same individual receiving prostaglandin E1. Our preliminary findings suggest that penile blood prostacyclin may participate in the pathogenesis of arteriogenic impotence and priapism.