The morphological variation in the recognized landmarks for the atrioventricular conduction system was studied grossly in 94 hearts with atrioventricular septal defect, assessing 20 hearts with normal atrioventricular septation as a control. In all the hearts with intact atrioventricular septal structures, the tendon of Todaro demarcated the superior boundary of the triangle of Koch. In hearts with atrioventricular septal defect, however, the landmarks for the conduction axis made up a separate nodal triangle. The tendon of Todaro, along with a bridging tendon not found in the normal heart, were variably developed in hearts with atrioventricular septal defect and formed a further triangle unrelated to the axis for atrioventricular conduction. The opening of the coronary sinus was also variable in its location and size. It was the location of the inferior bridging leaflet as it crossed the ventricular septum that was the best surgical landmark to the site of penetration of the atrioventricular conduction axis.