Sixty patients undergoing coronary artery bypass grafting operations with cold potassium cardioplegia as the method of myocardial preservation either received low-dose oral propranolol (10 mg every 6 hours; 28 patients) or served as controls (32 patients). The study period began after extubation and ended at the time of hospital discharge. On the fourth postoperative day, 24-hour Holter monitoring was performed to assess additional subtle differences in arrhythmias. The overall incidence of symptomatic postoperative arrhythmias was 31% in the control group: 6 patients (19%) had atrial fibrillation or flutter and 4 patients (12%), ventricular arrhythmias. By contrast, 1 patient (4%) in the propranolol group had atrial fibrillation, and no patient had ventricular arrhythmias. The difference in overall arrhythmia rates between the two groups is significant (p less than 0.025). Twenty-four-hour Holter monitoring demonstrated no additional differences in the frequency of simple or complex atrial or ventricular ectopy between the two groups. We conclude that the incidence of postoperative arrhythmias following coronary artery bypass operation is diminished by the oral administration of prophylactic low-dose propranolol. When compared with our previous study [1], in which the method of myocardial preservation was intermittent aortic cross-clamping and moderate hypothermia, there is no difference in the overall incidence of postoperative arrhythmias.