1. In nineteen patients, five with unilateral renal artery stenosis and fourteen with essential hypertension (WHO grades I–II), blood pressure, plasma and extracellular fluid volumes and plasma renin activity were studied at the end of three sequential periods: (a) after at least 3 days on a 60 mmol Na+ diet; (b) after 3 days of salt depletion induced with a diuretic and sustained on a 20 mmol Na+ diet; (c) after 3 days during which the 20 mmol Na+ diet was continued and beta-receptor blockade was induced by increasing dosages of propranolol up to 320 mg daily.
2. After sodium depletion extracellular fluid volume and plasma volume decreased and plasma renin activity increased; blood pressure did not change significantly.
3. After adding propranolol, plasma volume and extracellular fluid volume remained low, and there was a significant decrease in plasma renin activity and blood pressure.
4. No correlation could be demonstrated between changes of blood pressure and plasma renin activity.
5. When the responses of the five patients with renal artery stenosis were compared with those of the fourteen patients with essential hypertension, no significant differences were found.
6. Propranolol has a strong anti-hypertensive effect after Na+ depletion, irrespective of the absolute activities of plasma renin.