Cardiac arrhythmias were produced in the dog by the injection of epinephrine during the inhalation of 26 per cent cyclopropane in oxygen. Cardiac arrhythmias were usually associated with a rise in arterial pressure, but they were also seen with no change or decrease in blood pressure. The beta adrenergic blocking agent propranolol (0.3 mg/kg) caused transient but significant decrease in blood pressure and heart rate, increased the pressor response to epinephrine, abolished or diminished the tachycardia following epinephrine, and ccnsistentiy increased the arrhythmia threshold dose of epinephrine to 12 times that of control. Lidocaine (1.0 mg/kg) did not significantly alter the blood pressure or heart rate, moderate the tachycardia following epinephrine, or consistently increase the arrhythmia threshold dose of epinephrine. From the available evidence together with the authors' it seems reasonable to conclude that the mode of antiarrhythmic action of propranolol differs from that of lidocaine and that the anesthetic-catecholamine cardiac arrhythmias are due to stimulation of beta adrenergic receptors.