The beta-adrenoceptor blocking effects of orally administered atenolol on tachycardia induced by intravenous isoprenaline or by exercise have been studied in normal volunteers, and compared with the effects of similar doses of propranolol. The blood levels of atenolol at various times after oral administration were determined by g.l.c. and correlated with the degree of inhibition of tachycardia. Atenolol was shown to be a beta-adrenoceptor blocker in man, as in animals, in that it antagonized the chronotropic effects of isoprenaline and of exercise. The inhibitory effect of atenolol on exercise-induced tachycardia was evident at a concentration in blood of 0.2 mug/ml and virtually complete at 0.5 mug/ml. Higher concentrations than this did not produce significantly greater blockade. The effects of atenolol on exercise-induced tachycardia were similar to those of propranolol but it was less effective in blocking the rise in heart rate and fall in diastolic blood-pressure induced by intravenous infusion of isoprenaline. This separation of effects is considered characteristic of drugs causing preferential blockade of cardiac beta-adrenoreceptors. The half-life of atenolol in blood was calculated to ablut 9 hours.