1. Plasma renin activity (PRA) and renin dependency of the blood pressure was analysed in ten patients with various forms of hypertension before and during treatment with volume depletion and/or propranolol. Renin dependency was tested by infusion of the specific competitive angiotensin II antagonist Sar1-Ala8-angiotensin II (P113).
2. The P113-induced fall of the blood pressure did correlate with the log PRA (r = 0·888, P < 0001). This correlation was found irrespective of different types of hypertension and treatment schedules.
3. During volume depletion, PRA was stimulated and renin dependency of the blood pressure increased. Propranolol therapy suppressed PRA during normovolaemia as well as during volume depletion, and this was accompanied by a decrease of the renin dependency.
4. No indication was found that a given PRA is of special importance for blood pressure elevation in different patients.
5. Suppression of PRA by propranolol is one of the anti-hypertensive mechanisms of this drug.