Inquiry was made into the theory that closure of the efferent vein from the corpora cavernosa is essential for erection of the human penis. To determine whether the venous closure is indeed a prerequisite to human penile erection, two tests were carried out in men: (1) direct infusion in 133Xe into corpora cavernosa and (2) performance of carvernosography. In each case, penile erection was induced by providing the subject with sexual stimulation. The behavioral changes were studied through the 133Xe clearance curve and the contrast medium, respectively. When the penis remained flaccid, the 133Xe clearance curve followed a gentle path and the contrast medium could be noted within the penis for a relatively long period. However, on erection with sexual stimulation, the 133Xe clearance curve fell rapidly instead of following the gentle course expected in the case of venous closure. Also, the contrast medium quickly flowed out of the corpora cavernosa. The human penis therefore can well erect without closure of the efferent vein from the corpora cavernosa.