The comparative antiarrhythmic activity of quinidine, propranolol and the combination of these two drugs was studied in experimental atrial and ventricular arrhythmias in the dog. Quinidine, but not propranolol, suppressed atrial arrhythmias produced by topical application of aconitine to the atrium, as well as the ventricular arrhythmias that developed approximately 20 hr following coronary artery ligation. When the two drugs were coadministered, synergism occurred in the atrial arrhthmias but not in the ventricular arrhythmias. Some possible mechanisms of action of these drugs are discussed. The change in response to quinidine by coadministration with propranolol may have resulted in part from suppression of atrial automaticity in the aconitine experiments and from further slowing of ventricular conduction in the coronary artery ligation experiments. Both effects might have occurred as a result of myocardial beta adrenergic receptor blockade.