1. Propranolol, saralasin and captopril changed blood pressure in normotensive as well as hypertensive subjects.
2. The percentage change in blood pressure with these three drugs for a given plasma renin activity was similar in normotensive and hypertensive subjects.
3. This suggests that when the renin-angiotensin system is maintaining blood pressure, it maintains the blood pressure to the same extent in percentage terms in normotensive and hypertensive subjects for a given plasma renin activity.
4. Saralasin has marked agonist activity, and probably underestimates the participation of the renin—angiotensin—aldosterone system in the maintenance of blood pressure. The fall in blood pressure that occurred with captopril in normal subjects on their normal sodium intake suggests that the renin—angiotensin—aldosterone system may have an important role in the control of blood pressure in normal subjects on their normal sodium intake. If it does, our results suggest that the renin—angiotensin—aldosterone system plays no greater role in maintaining blood pressure in patients with essential hypertension than normotensive subjects for a given plasma renin activity.