One hundred consecutive patients requiring propranolol hydrochloride before undergoing isolated aortocoronary bypass procedures were examined. In half the patients, propranolol therapy was discontinued, whereas the other half continued to receive intraoperative and postoperative propranolol regardless of clinical events. Although there were no preoperative differences in the apparent degree of coronary arterial disease or left ventricular function in the two groups, postoperative supraventricular arrhythmias were less frequent in the propranolol-treated group, most noticeably in those receiving less than 320 mg preoperatively. In patients who had received large preoperative doses (greater than or equal to 320 mg/day), there were no significant differences in postoperative supraventricular tachycardias. Continued propranolol therapy following isolated coronary bypass surgery appears to be a safe and efficacious method of decreasing the incidence of postoperative supraventricular tachycardias.