This is a test in the investigation of male impotence. It differentiates impotence due to vascular causes from psychological causes. A device attached to the penis records the frequency and rigidity of erections in sleep. In sleep subjective control is diminished. A man who is impotent due to vascular causes will not have erections in sleep. However, one who is impotent because of psychogenic reasons are more likely to have erections in his sleep.