Closing the ventricular septal defect in patients with atrioventricular discordance with sutures placed through the defect onto the morphologically right septal surface should avoid production of complete heart block. To discover whether this procedure otherwise affects conduction, standard electrocardiogram (ECG) were compared preoperatively and postoperatively in 11 such patients. Operation had lengthened the PR interval in one and shortened it in another. The QRS interval remained unchanged in 5 patients thought 2 of these showed minor changes in QRS configuration. Four patients showed QRS prolongation (increase greater than 20 msec) with delayed conduction towards the right ventricle and the initial QRS vector preserved, i.e., morphologically right bundle branch block. Two patients showed QRS prolongation with initial QRS vector alteration and delayed conduction towards the left ventricle, i.e., morphologically left bundle branch block. Repair through the morphologically left ventricle invariably caused bundle branch block. Repair through a right atriotomy caused bundle branch block in only 3 of 8 patients. Therefore closure of a ventricular septal defect by this method, particularly transatrially, need not affect conduction. Morphologically left ventriculotomy rarely if ever causes morphologically left bundle branch block. When present, morphologically right bundle branch block presumably results from interruption of the proximal right bundle.