Hypertensive patients with low plasma renin activity are suspected of having mineralocorticoid excess. We treated 26 hypertensive patients with the mineralocorticoid antagonist, spironolactone, 200 mg daily, and 25 patients with hydrochlorothiazide, 100 mg daily, for 12 weeks. Patients with low renin activity (below 250 ng/100 ml, 10 mEq sodium diet, upright position) and low normal renin activity (250 to 400 ng/100 ml) responded to both drugs with a significantly larger fall in blood pressure than was achieved by patients with renin activity above 400 ng/100 ml. Fall in blood pressure was related to the initial weight loss induced by spironolactone (r =.51), but was not related to the weight loss induced by hydrochlorothiazide. Both spironolactone and hydrochlorothiazide are more effective in reducing blood pressure in patients with low and low-normal renin activity than in other hypertensive patients.