When plasma renin activity is related to sodium balance as evaluated from urinary sodium excretion, a physiological index of normality is obtained along with the ability to detect subtle variations in renin secretion. This has made it possible to correlate more closely the antihypertensive action of propranolol with the renin level. Propranolol reduced renin in all patients but did not lower blood pressure in low renin hypertensive states. It was dramatically effective in lowering blood pressure of high renin and some normal renin patients. Its antihypertensive effect was directly related to the pretreatment renin level and also to the absolute decrement in renin reduction achieved. This was true in malignant, renal, renovasular, and essential hypertension. Altogether, these findings point to the involvement of the renin-aldosterone axis in these hypertensive states. They suggest that a major component of the antihypertensive action of propranolol is consequent to an antirenin effect which is perhaps neurologically mediated. Accordingly, in addition to its therapeutic values, the response to propranolol may provide a pharmacological indicator of the renin involvement in essential hypertension and it also may be a useful adjunct for predicting surgical curability of renovascular hypertension.