Penile blood flow (PBF) was measured by the Xe-133 clearance method. In 16 patients, complete erection was obtained by visual sexsual stimulation (VSS). Their mean PBF before and after erection were 1. 95 ml/100 g/min and 10. 71 ml/100 g/min, respectively. In 69 patients, full erection was obtained by intracavernous injection of 20 µg prostagrandin El (PGE1). Their mean PBF before and after administration were 2. 00 ml/100 g/min and 6. 28 ml/100 g/min, respectively. The mechanism of erection by

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... intracavernous PGE1 was different from that of physiological erection, since the mean PBF after PGE1 was apparently lower than VSS. In diabetic and geriatric patients, the mean PBF before PGE1 was lower than that of cerebrospinal disorder patients. This result indicated that arterial lesion was the main cause of impotence in diabetic and geriatric patients. In some patients, while mean PBF after PGE1 injection was considered to be sufficient, full erection was not obtaind. A venous impotence was suspected in such cases. Thus, vasculogenic impotence could be diagnosed by investigating PBF before and after PGE1 injection.