The antihypertensive effect and side-effects during 12 months' treatment with bendroflumethiazide and propranolol have been compared in two randomly selected, equally large groups (n= 53) of previously untreated male hypertensives. Systolic BP above 170 or diastolic BP above 105 mmHg on two occasions were defined as hypertension. The same BP reduction was achieved in both groups. During the 12 months' treatment one subject on bendroflumethiazide developed diabetes mellitus and one on propranolol developed cardiac decompensation. None developed gout. Contrary to what had been presumed, glucose tolerance improved during 12 months' treatment with both agents, while there were no changes in fasting blood sugar, insulin or triglyceride concentrations. No changes were found in serum potassium or total body potassium during 12 months' bendroflumethiazide treatment, while serum potassium increased during treatment with propranolol. Uric acid increased slightly during treatment with both agents. Prolongation of the follow-up to 24 months did not change any of the findings regarding metabolic changes during treatment. The frequency of subjective side-effects decreased to the same extent during treatment with both drugs. It is concluded that bendroflumethiazide and propranolol are equally useful as antihypertensive agents and that the risk of impariment of glucose metabolism and potassium balance seems to be very slight during treatment with bendroflumethiazide in mild hypertension.