47 years). There was an age-related increase in the proportion of total sleep in rapid eye movement (REM), which was only apparent in the older age group. In the volunteers, there was a curvilinear dose-response effect on NPT. The lower dose modestly increased erectile response, particularly during non-REM sleep, whereas the higher dose reduced erectile response, most noticeably during REM. The only positive effect of the higher dose was an increase in spontaneous erections after lights out before sleep onset. The older dysfunctional group showed no drug effects on NPT. The younger dysfunctional men showed increased erectile response during non-REM with the higher dose, an effect that was also significant in the interval between sleep onset (Stage 2) and first REM.