The effects of trazodone on the nocturnal penile erection were demonstrated in 14 impotent patients by Rigiscan recordings. Trazodone significantly increased the total erectile time, per cent of sleep with erection and maximum rigidity, while it had no effect on increment of tumescence size. Trazodone (150 mg to 200 mg) was administered to 35 clearly organic impotent patients in a 5-week single-blind design. Patients were evaluated by questionnaire. The study showed an improvement in 68.6 per cent of the patients receiving trazodone versus 11.4 per cent receiving placebo. In vitro, trazodone could relax rabbit corporeal smooth muscle contractions induced by exogenous norepinephrine and inhibit contractions elicited by transmural electrical stimulation of adrenergic nerve endings. The effects of trazodone on penile erection may be due to a peripheral alpha adrenoceptor antagonism and central unknown mechanism. We conclude that trazodone may prove an effective treatment for impotent patients.