Cardiac arrhythmias were produced in the dog by injections of epinephrine or isoproterenol during intravenous thiopental or propanidid anesthesia. Propanidid did not increase the pressor response to epinephrine, reduce the depressor response to isoproterenol, or diminish the tachycardia following epinephrine or isoproterenol. Propanidid, however, consistently increased the arrhythmia threshold doses of epinephrine and isoproterenol four times those during thiopental anesthesia. It is concluded, therefore, that the antiarrhythmic effect of propanidid is not related to beta adremergic blockade but to its local anesthetic or quinidine-like action and that propanidid is an excellent intravenous anesthetic in view of its antiarrhythmic action on the heart.