1. Plasma renin activity and circulating amounts of blood angiotensin II were measured in twenty-six patients with uncomplicated essential hypertension.
2. Measurements were made during a control period and at 1, 4, 9 and 14 weeks of diuretic treatment with metolazone or hydrochlorothiazide.
3. Linear regression analysis indicated a progressive change in the relation of plasma renin activity and blood angiotensin II.
4. During chronic diuretic therapy (beyond 14 weeks) blood angiotensin II had stabilized to low and relatively fixed values across a wide range of plasma renin activities.