To determine whether halothane and morphine, commonly used during anesthesia for cardiac operations, potentiate the beta blocking activity of propanolol, hemodynamic changes induced by five incremental doses of propranolol (10, 20, 50, 120, 200 microgram/kg) were measured during halothane, 1 per cent, in oxygen, and morphine, 4 mg/kg. Against a background of contant beta stimulation by infusion of isoproterenol, 0.1 microgram/kg/min, and vagal blockade by atropine, 3 mg, propranolol produced significant dose-related decreases in heart rate, cardiac index, stroke volume index, and left ventricular dp/dtmax and significant increases in mean aortic pressure, systemic vascular resistance, and pulmonary capillary wedge pressure. Compared with basal anesthesia with pentobarbital, 15 mg/kg, neither morphine nor halothane increased sensitivity to any measured effect of propranolol expressed as the slope of the log dose-response relationship. It is concluded that the beta blocking activity of propranolol is not potentiated by morphine and halothane anesthesia but, rather, their effects are additive.