The effects of penbutolol 20-30 mg twice daily and of metoprolol 50-150 mg three times daily were studied in five and nine patients, respectively, with moderate hypertension. Both drugs significantly reduced blood pressure and pulse rate under basal conditions and in connection with exercise. Plasma catecholamine levels were not altered in supine or upright position, neither during penbutolol nor during metoprolol treatment. Following work there was an enhanced response in both plasma noradrenaline and adrenaline during penbutolol but only in plasma noradrenaline during metoprolol treatment. Both drugs effectively reduced basal plasma renin activity but only penbutolol abolished the renin response to work. Penbutolol significantly reduced urinary aldosterone excretion whereas metoprolol caused no significant changes. Neither penbutolol nor metoprolol affected maximal working capacity.