Saralasin, a specific competitive inhibitor of angiotensin II, was administered in a controlled, prospective study designed to test the hypothesis that this agent is a useful tool for the detection of renovascular hypertension. 13 patients, 11 with renovascular hypertension and 2 with high-renin essential hypertension, showed a gross, readily apparent decrease in blood pressure after receiving saralasin. 8 patients with essential hypertension and normal or low renin levels exhibited no depressor response to the drug. In the patients with renovascular hypertension, blood pressure response during angiotensin blockade compared favourably with renal vein renin determinations as a predictor of operative results. Because saralasin testing has resulted in few if any falsely positive or negative results when considered as a diagnostic procedure for renin-mediated hypertension, and because it is safe, it may become an ideal initial screening procedure. The saralasin test (either bolus injection or sustained infusion) is completely valid only if the patient is mildly salt-depleted, is not taking other antihypertensive medication, and is genuinely hypertensive at the time of the test.