Normal atrioventricular conduction is dependent upon proper functioning of the specialized conducting system within the heart. This system may be subjected to injury during the repair of congenital cardiac defects from: (1) sutures used in repairing the defect; (2) direct trauma from retractors and other instruments; (3) stretch and/or injury to the moderator band, and (4) interruption of the blood supply. One of the methods which may be used to demonstrate the anatomical relationship of the specialized conducting system to a cardiac defect at the time of surgery is to outline the course of this system at the time of operation. This is one of the problems which this laboratory has been studying during the past 3 years.1-4 By utilizing the heart-lung machine any chamber of the heart may be opened, and surface electrodes may be positioned at specific points under direct vision. It is possible to localize and