To determine whether the elevated plasma renin activity in some cases of mild essential hypertension expresses sympathetic-nervous-system over-activity, we compared indexes of sympathetic activity in 16 patients with mild high-renin essential hypertension, 15 hypertensive patients with normal plasma renin activity and 20 normal subjects. Patients with elevated activity exhibited a raised plasma norepinephrine concentration (P less than 0.05), a greater fall in cardiac output with cardiac beta-adrenergic blockade by intravenous propranolol (P less than 0.01), reduction in total peripheral vascular resistance with alpha-adrenergic blockade produced by intravenous phentolamine (P less than 0.01), and reduction to normal of blood pressure by "total" autonomic blockade (atropine, propranolol and phentolamine). On psychometric testing, patients with high-renin hypertension, but not those with normal plasma renin activity, exhibited suppressed hostility (P less than 0.01), a behavioral pattern linked to increased sympathetic activity. The hypertension in these patients with high renin activity is neurogenic and possibly psychosomatic in origin.