The relationship between renin responsiveness to furosemide and the antihypertensive effect of captopril in patients with normal-renin essential hypertension were studied in 23 patients including nine men (mean age, 41 years) and 14 women (mean age, 40 years). Those who had an increment of more than 50% in plasma renin activity (PRA) two hours after an intravenous injection of 20 mg furosemide were classified as group A (n = 13) and the others were classified as group B (n = 10). Baseline PRA, plasma aldosterone and mean blood pressure (MBP) were not different between the two groups. Both groups showed no significant difference in natriuresis following furosemide administration. Significant change in MBP was observed after an oral dose of 100 mg captopril within four hours in group A, but not in group B. These data suggest that renin responsiveness to a single intravenous dose of furosemide can be a useful test for predicting the therapeutic response to captopril in patients with normal-renin essential hypertension. The furosemide test had a sensitivity of 75%, a specificity of 64%, a positive predictive value of 69% and a negative predictive value of 70%.