Hydrochlorothiazide in a daily dose of 150 mg and spironolactone in a daily dose of 400 mg (100 mg of the currently available preparation) were found to have equivalent antihypertensive effects—reducing basal blood pressure, decreasing the pressor response to levarterenol bitartrate (norepinephrine) and angiotensin amide, and increasing the depressor response to a ganglion-blocking agent, trimethaphan camsylate—in a double-blind study of 11 hypertensive patients. The two diuretics, when given together, appear to act independently and additively in their antihypertensive effects. Serum potassium level, lowered by hydrochlorothiazide, was in the normal range during administration of spironolactone alone or in combination with hydrochlorothiazide. Spironolactone can thus be a useful substitute for a benzothiadiazine derivative in the treatment of hypertension. The administration of a combination of these diuretics, a thiazide and an aldosterone antagonist, can render hypertension more readily manageable.