The material comprised the data of 146 patients with tetralogy of Fallot treated by corrective intracardiac surgery. Electrocardiograms were recorded in all cases before and in 108 cases after operation. The majority of the patients were between 5 and 20 years of age at the time of surgery and have been followed for an average of 1.5 years.Arrhythmias, mostly extrasystoles, were recorded in eight cases before operation and in 14 cases on the last follow-up examination. The amplitude of the P wave in lead II decreased by 1 mm. or more after surgery in one third of the cases.Seventeen patients developed complete AV block postoperatively, seven died; of the 10 who survived operation, in all but two the electrocardiogram reverted to normal sinus rhythm.The electrocardiogram showed right axis deviation and right ventricular hypertrophy in the majority of cases. A balanced axis and signs of combined ventricular hypertrophy were commonest in acyanotic patients and more common in cyanotic patients who had had previous shunt procedures than in cyanotic patients who had had no previous shunt. Two patients showed electrocardiographic signs of left ventricular hypertrophy. After corrective cardiac surgery, the mean electrical axis and the pattern of the QRS complex in V1and V6changed toward normal in the eight patients in whom intraventricular conduction defects did not appear and in the 26 patients with incomplete right bundle-branch block.Two of nine surviving patients in whom a small coronary artery was divided at surgery developed the pattern of myocardial infarction in the postoperative electrocardiogram.Conduction disturbances frequently appeared during operation; they were considered to be caused mainly by direct trauma to the conduction system during closure of the ventricular septal defect or during infundibulum resection. Complete right bundle-branch block appeared in 74 (69 per cent) and incomplete right bundle-branch block developed in 20 (19 per cent) of surviving patients. Only eight patients showed no disturbances of conduction after surgery. The right bundle-branch blocks persisted throughout the follow-up period. The conduction disturbances have not adversely affected the postoperative course, nor have they caused any discomfort to the patients.