Labetalol (Trandate) is a new antihypertensive agent with both alpha- and beta-adrenoceptor blocking properties. In a double-blind cross-over study the antihypertensive action and side-effects of labetalol and propranolol were compared in 18 previously untreated outpatients with hypertension, WHO stage I--III. Mean daily dose of labetalol was 667 mg and of propranolol 129 mg. Labetalol reduced systolic and diastolic blood pressure in the seated and upright position significantly more than propranolol. The pulse rate reduction was greater with propranolol. Side-effects were more pronounced with propranolol. The antihypertensive effect, effect on pulse rate and pharmacokinetics of a single oral dose of 400 mg labetalol were studied in 6 patients with normal and 6 patients with impaired renal function (creatinine clearance less than 20 ml/min), all belonging to WHO stage I--II. A significant fall in pulse rate and systolic and diastolic blood pressure was observed in both groups, the duration being more than 25 h. No difference was found between the two groups. From the serum concentration-time curves the elimination rate constant, elimination half-life and area under the curve were calculated. The mean values of the two groups did not differ significantly. A pronounced interindividual variation was found in both groups.