The location and size of the coronary sinus in hearts with atrioventricular septal defect were investigated in relation to the known disposition of the atrioventricular conduction axis. We examined the morphology in 40 hearts and supplemented this series with two other hearts that had been serially sectioned previously. The coronary sinus received drainage from a persistent left superior caval vein in 5 hearts. Six cases of 40 had malalignment of the septal structures relative to the crux of the heart. In these, the conduction axis was anticipated to course in the position where the inlet ventricular septum met the atrioventricular junction. The coronary sinus terminated in the left atrium in 4 hearts: 2 in the morphological series and 2 that were sectioned for histological studies. The sectioned hearts showed the atrioventricular conduction axis in the usual position for the defect, unrelated to the coronary sinus. The principle that the node and penetrating bundle are located at the intersection of the ventricular septum with the atrioventricular junction holds good despite the variability of the coronary sinus.