The antihypertensive effect of the new beta-blocker, ICI 66,082 was studied in 37 patients with renal or essential hypertension. A daily dose of 75 mg produced a significant antihypertensive effect and a further blood pressure decrease was obtained by increasing the daily dose up to 300 mg; at this treatment level, the antihypertensive effect averaged 29.4/24.0 mm Hg. Increasing the dose from 300 to 600 mg daily did not produce a significant additional blood pressure decrease. Six of the 37 patients retained fluid, producing a loss in the antihypertensive effect. No significant correlation was found between the percentage systolic blood pressure fall and the following parameters determined before therapy: systolic blood pressure and its variability, heart rate (at rest and after stimulation by exercise or isoproterenol), cardiac output and total peripheral resistance, renin concentration (at rest and after stimulation) and urinary excretion of catecholamines and derivates.