The major morphologic features of the atrioventricular ( AV) conduction system of the human heart have been discussed for several years -from time to time, even the anatomic existence of the system has been questioned. In order to familiarize ourselves with the morphologic features of this system and to assess points of disputation before investigating it in relation to congenital ventricular septal defects, we reviewed the literature relevant to the human heart and carried out the studies, reported herein, of essentially normal human hearts. Attention was focused on the major divisions of the AV conduction tissue. As this was a morphologic study, we made no attempt to demonstrate the physiologic significance of the structures, collectively called the atrioventricular conduction system.