The effects of inhibition of the renin-angiotensin system with quinapril on adrenergic transmission were determined in 18 patients with moderate essential hypertension. Plasmatic renin activity, catecolamines, and aldosterone as well as vascular reactivity after norepinephrine and angiotensin II perfusion were determined before and after treatment with the ACE inhibitor quinapril up to 40 mg/day during four months. In the responder patients (patients with a casual diastolic blood pressure decrease > 5 mm Hg determined at the beginning of the second vascular reactivity study, (N = 9) a decrease of plasma norepinephrine and epinephrine and a slight increase of vascular reactivity was found. Non-responder patients (N = 9) did not show any significant changes in these parameters. These results demonstrate that the hypotensive effect of quinapril may be related not only to the inhibition of RAS, but also to the blockage of the adrenergic system.