Abstract Combining propranolol and quinidine should improve the antiarrhythmic action of quinidine by blocking beta adrenergic receptors, an action helpful in cases of arrhythmias associated with anesthetic agents, digitalis and stress. To test this hypothesis, 60 patients with paroxysmal arrhythmias resistant to quinidine and 17 with chronic atrial fibrillation were given propranolol alone or in combination with quinidine (or procainamide) and followed up 1 to 17 months. Patients with severe aortic valve disease, severe mitral insufficiency, acute myocardial infarction or obstructive airway diseases were excluded from this study. The condition of 13 of 19 patients with paroxysmal atrial or nodal tachycardia was improved. In paroxysmal ventricular tachycardia combined therapy or propranolol alone was effective in 11 of 14 patients. Paroxysmal atrial flutter or fibrillation was suppressed in 11 of 16 patients, with a 75 percent or greater reduction in attacks. Prophylaxis of chronic atrial fibrillation terminated by drugs or by electroshock was effective in 9 of 11 patients (15 trials) followed up for 7 months. Sustained atrial fibrillation was terminated with propranolol, 40 to 160 mg, and quinidine sulfate, 1.2 to 1.6 g/day, in 41 percent of patients, a result no better than with propranolol alone. Propranolol slowed the ventricular rate in normal sinus rhythm as well as in atrial fibrillation and flutter. Significant slowing, although it was less in magnitude, occurred with the combination of propranolol and quinidine. It is concluded that propranolol potentiates quinidine and allows effective prophylaxis of atrial, nodal and ventricular arrhythmias in quinidine-resistant patients. Diarrhea was the only side effect peculiar to the combination of drugs.