Abstract The conduction systems of 9 hearts with ventricular septal defect coming to autopsy after open heart surgery were studied by serial section. In 2 cases severe A-V block was present; in 2, complete right bundle branch block; and in the remainder there was no clinical evidence of A-V block after surgery. The severe A-V block in both cases could be ascribed to injury of the penetrating portion of the A-V bundle. An alternate explanation in 1 case was the injury to the branching bundle and the posterior radiation of the left bundle branch. The right bundle branch block in 2 cases for which a postoperative electrocardiogram was available was associated with severe injury to the right bundle. In double outlet right ventricle with an anterior ventricular septal defect, the branching portion of the A-V bundle may lie considerably to the left of the summit of the ventricular septum as in some cases of tetralogy. In a case of double outlet right ventricle with a massive posterior ventricular septal defect, the conduction system resembled that of common A-V orifice or single ventricle.