Twenty-two patients manifesting arrhythmias refractory to conventional therapy have been treated with propranolol 1 to 19 days after cardiac surgery. Rapid control of ventricular response to supraventricular arrhythmias, abolition of ventricular irritability caused by digitalis excess, and frequent conversion of acute atrial fibrillation or flutter were attained. Control of the arrhythmia was achieved in all but one instance. No significant side effects were encountered, although the potential risks of propranolol are stressed. Propranolol appears to fill specific needs unsatisfied by standard therapy for arrhythmias developing after cardiac surgery.