Electrophysiologic delineation of the atrioventricular conduction system at surgery is described in two patients with corrected transpostion of the great arteries in situs inversus. Intra-atrial electrograms were recorded in one patient from sites immediately adjacent to the coronary sinus located in the left-sided right atrium. The intraventricular portion of the atrioventricular conduction system was identified in both patients along the posterior and inferior margin of the ventricular septal defect, in contrast to the superior and anterior location found in corrected transposition of the great arteries in situs solitus. In contrast to the superior and anterior location found in corrected transposition of the great arteries in situs solitus. The course of the conduction system in the hearts of these two patients and a possible relationship to the cardiac loop and dual origin of the atrioventricular node is discussed. These cases illustrate the usefulness of segmental diagnosis of congenital heart disease and of electrophysiologic identification of the specialized atrioventricular conduction system at surgery.