Corpus cavernosum pressure in man during erection has not been measured previously with a noninvasive technique. Using such a technique it has been shown that during erection in young men intracavernosal pressure has peaks of at least ten times higher than the systemic systolic pressure. These findings indicate that during erection the intracavernosal circulation is contained, at least partially, within a closed system, separated from the general circulation. Also erection cannot be the result of circulatory function only.