To identify patients with low-renin hypertension, we measured plasma renin activity after the administration of 40 mg of furosemide intravenously and 30 minutes of upright posture in 127 normotensive subjects and 363 patients with essential hypertension. Plasma renin activity 30 minutes after intravenous furosemide was found to be closely correlated to the level found after either 2 or 4 h of standing or 3 days of a low-salt diet plus 2 h of upright posture. Renin responsiveness was significantly lower in hypertensive patients, blacks, and women, compared with normotensive subjects, whites, and men respectively. The level of plasma renin activity in most normal white subjects was greater than 1.0 ng/ml - h and in most normal blacks was greater than 0.5 ng/ml - h. It was below those levels in 23% of white hypertensive and 25.2% of black hypertensive patients respectively. The mean level of plasma renin activity fell with increasing age of hypertensive patients. This procedure is recommended as a safe, easy, and reliable test for assessing renin responsiveness and identifying the low-renin state.