Effects of two dosage strengths of hydrochlorothiazide and triamterene in combination and chlorthalidone on blood pressure and serum potassium were compared in 126 patients in a double-blind, placebo-controlled study. Each drug regimen was administered once a day. All three regimens had significantly reduced blood pressure at each of the eight weeks of therapy. Compared to chlorthalidone, the lower dosage of hydrochlorothiazide/triamterene usually produced smaller decreases in blood pressure, while the higher dosage of the combination usually produced decreases of equal magnitude. The incidence of clinically significant hypokalemia, ie, serum potassium < 3.0 mEq/liter, was 0% for both dosage strengths of hydrochlorothiazide/triamterene and 15% for chlorthalidone. There were no significant differences among regimens in incidence of clinical adverse effects. These data indicate that the hydrochlorothiazide/triamterene combination can be used effectively on a once-a-day basis in the treatment of hypertension.