Eighty-five patients receiving long-term propranolol therapy were randomized after aorta-coronary bypass grafting either to receive minidose propranolol (Group I) or to serve as controls (Group II). They were compared with 18 patients (Group III) who did not receive beta blocking agents prior to operation but were given propranolol postoperatively. Poor-risk patients (those having left ventricular aneurysms, low ejection fraction, or congestive heart failure) as well as patients who required catecholamines postoperatively were included in the study. All three groups were comparable with respect to all risk factors. Propranolol (5 to 10 mg/6 hr) was started through a nasogastric tube 6 hours after operation and continued orally in all patients in Groups I and III. Supraventricular tachyarrhythmia appeared in two of 37 patients in Group I (5%), 19 of 48 patients in Group II (40%), and five of 18 patients in Group III (27%). The incidence of supraventricular tachyarrhythmia was significantly lower in Group I than in Groups II and III (p less than 0.001, Group I versus Group II; p less than 0.01, Group I versus Group III). In conclusion, low-dose propranolol is very effective in preventing supraventricular tachyarrhythmia following aorta-coronary bypass in patients receiving beta blockers preoperatively. The increased tendency for postoperative supraventricular tachyarrhythmia to develop in these patients is attributed to hypersensitivity to adrenergic stimulation after propranolol withdrawal. The tachyarrhythmia can be prevented by early reinstitution of propranolol in low doses after the operation.