On a normal diet, hydralazine hydrochloride (20 mg) given intravenously caused significant increase in plasma renin activity in five of seven normotensive subjects and ten of 20 patients with essential hypertension. Seven patients with renovascular hypertension responded to hydralazine with much greater increase; the overlap in predrug renin activity between patients with essential and renovascular hypertension diminished after hydralazine. Three-day sodium restriction enhanced the effect of hydralazine in a majority of patients with essential hypertension; the postdrug renin activity exceeded 8 mμg/ml in 11 of the 12 patients. In all four patients with primary aldosteronism renin activity remained undetectably low even after hydralazine combined with sodium restriction. The findings suggest that the hydralazine test might be useful in the differential diagnosis of hypertension. However, the drug should be used carefully because of untoward side effects.