A low dosage of the specific MAO-A inhibitor clorgyline (5-10 mg/day) was administered chronically to 10 depressed patients. Peripheral noradrenergic activity, as assessed by measurements of the plasma norepinephrine (NE) concentration, heart rate, and blood pressure, was determined before and after drug treatment. The administration of a low dosage of clorgyline was associated with a decrement in peripheral presynaptic noradrenergic activity. Comparison with the results of similar studies of the effects of desmethylimipramine (DMI) suggests that antidepressant drugs with unrelated primary actions affect peripheral noradrenergic functions differently.