The effect of (-)-deprenyl, a rapidly acting selective monoamine oxidase (MAO) B inhibitor, on the sleep and mood of six healthy young male adults was investigated. The drug was administered double-blind in a balanced cross-over design. The dose (5-10 mg/day for 3 days) was chosen to cause complete inhibition of MAO, a process which usually takes 1-2 weeks with conventional MAO inhibitors. The inhibition was monitored by measuring platelet MAO activity and phenylethylamine excretion. Urinary phenylethylamine concentration was raised in all subjects. Subjects were unaware of any sleep disturbance due to the drug although the electroencephalogram (EEG) showed increased wakefulness. The onset of rapid-eye-movement (REM) sleep was delayed and the total amount reduced; the amount of stage 2 sleep was increased. The only effect of the drug on mood was to decrease the level of alertness prior to sleep. There was a slight but significant increase in the pre-sleep systolic blood pressure. There were no effects due to drug withdrawal.