Quinidine, propranolol and their combination were studied in atrial fibrillation induced by the topical application of acetylcholine or aconitine, injury-stimulation-induced atrial flutter and ventricular ectopic tachycardia produced by coronary occlusion in the dog. The effect of combination of quinidine and propranolol was significantly greater than that of the individual drug in atrial arrhythmias but not in ventricular arrhythmias. Support is provided for the combined clinical use of quinidine and propranolol in the treatment of atrial arrhythmias.