1. In a four period, double-blind, placebo-controlled study, apomorphine (0.009 mg kg-1 subcutaneous route) and yohimbine (0.30 mg kg-1 i.v. route) were administered to 10 male healthy volunteers. Penile circumference was monitored using plethysmography and subjective sexual arousal was self-assessed. Data were collected before, during and after a stimulation session during which 50 erotic slides were projected to the subjects. 2. Apomorphine induced an erection starting from the fourth minute post-injection, and potentiated the visually-induced response. Self assessment showed increased tumescence and rigidity without modifications of sexual arousal. 3. Yohimbine did not modify penile diameter without stimulation and did not affect the physiological response to erotic stimuli. 'Sexual excitement' was decreased by yohimbine during the post-stimulation phase. One subject experienced severe anxiety while he was infused with yohimbine. 4. These results are discussed with reference to animal data and to recent studies carried out in impotent patients.