A study has been conducted into the morphological arrangement of the atrioventricular junctional area of human heart. The area was investigated in infantile, child, young adult, middle-aged, and old nodes. Although marked individual and ageing variations were observed, a general pattern of nodal architecture could be distinguished. The junctional area was therefore divided into four areas: (1) a transitional zone, (2) the compact node, (3) the penetrating bundle, and (4) the branching bundle. The transitional zone was intermediate between atrial myocardium and compact nodal specialized myocardium, but itself exhibited specialized characteristics. An important connection of the zone was to the myocardium of the left side of the interatrial septum. The compact node was itself composed of two segments which approached each other anteriorly from the mitral and tricuspid extremities of the septal anulus fibrosus. The junction of compact node and penetrating bundle could not be distinguished using cytological criteria. It was arbitrarily defined as the last point to make contact with transitional cells. Tissue distal to this was considered as penetrating bundle. A bypass tract was defined as any fiber contacting the bundle distal to this point, but such tracts were not observed in normal hearts. The branching bundle originated at the point of bifurcation of the penetrating bundle.