Several aspects of beta-adrenoreceptor blockade in man can be investigated in healthy volunteers. Beta-adrenoreceptor blocking activity was investigated by measuring the influence on isoprenaline-induced tachycardia and on tachycardia due to exercise on the cycle ergometer. In these experiments pindolol was found to be 20 to 40 times more potent than propranolol. The partial agonist activity of pindolol is responsible for its small influence on heart rate under resting conditions, but probably also for its smaller effect on bronchial tone when compared with propranolol. A comparison of the duration of action of different drugs can be made by studying the effects on exercise-induced tachycardia. Two hours after oral administration, 5 mg of pindolol and 100 mg of propranolol were about equally active. The duration of action of pindolol, however, was clearly longer than that of propranolol. This should allow a single daily dose to be given to most patients for the treatment of hypertension. A comparison of beta-adrenoreceptor blocking activity after oral and intravenous administration provides information on the bioavailability of these drugs. In studies with pindolol the activities measured 2 hours after oral and 75 minutes after intravenous administration were about equal. This is in good agreement with the results of pharmacokinetic studies, which show almost complete absorption and a small first-pass effect of pindolol.