The magnitude and duration of effect of a single 40-mg oral tablet of oxprenolol on the tachycardias associated with motor-car driving, isoprenaline infusion, and walking were compared against placebo in six normal people by a double-blind study. The tachycardias due to driving and isoprenaline were both conspicuousy reduced for over eight hours; the magnitude and duration of the reduction in exercise tachycardia was substantially less. Thus relatively small doses of beta-receptor antagonists will suppress the increase in heart rate induced by mental stress or catecholamines with relatively little effect on the response to everyday exercise. Possibly smaller doses of these drugs would relieve emotionally-induced anginal pain and tachycardia.