After acute beta-adrenergic blockade (5 mg atenolol intravenously) in 11 patients with essential hypertension but no heart failure arterial blood pressure and inotropic state fell slightly (-5.4% and -7.5%, respectively), but there was a definite decrease in heart rate (-13.8%), cardiac index (-11.5%) and cardiac work (-14.3%). There was a marked decrease in coronary blood flow (-14.5%) and myocardial oxygen uptake (-13.6%), while the coronary arterio-venous oxygen difference remained normal. Coronary vascular resistance increased significantly (+12.7%). Atenolol increased the coronary reserve of the left ventricle by about 21% in the five patients in whom it was measured. The results indicate that during acute beta-adrenergic blockade in essential hypertension there is an effective lowering of the left ventricular systolic load, with an equivalent decrease in myocardial energy requirement. The change in coronary vascular resistance and increase in coronary reserve of the left ventricle during this blockade is apparently the result of metabolic changes.