The effects of 5 mg bendroflumethiazide plus 15 mmol potassium chloride versus 50 mg hydrochlorothiazide plus 5 mg amiloride on blood pressure and serum electrolytes were investigated in a 12-week, open, randomized study with parallel treatment groups. Forty-five patients with previously untreated hypertension and 33 hypertensive patients previously treated with thiazides until 1 month before allocation to the present treatments, completed the trial. Systolic and diastolic blood pressures were reduced significantly during treatment with both drug combinations. Significant negative correlations between initial serum potassium levels and the thiazide-induced decrease were observed. Mean potassium concentrations decreased by 0.39 mmol/l with bendroflumethiazide/potassium chloride and by 0.30 mmol/l with hydrochlorothiazide/amiloride. It is concluded that both treatments were effective and well tolerated in controlling patients with mild to moderate hypertension, and that the difference in the mean serum potassium concentration between the two treatments was not statistically significant.