The specialised tissues responsible for the generation and conduction of the cardiac impulse consist of the sinus node, atrioventricular conduction axis, and the Purkinje network. Further areas of specialised tissue that are discrete from the atrioventricular conduction axis are to be found within the atrioventricular junctions. In animal hearts, it has been shown that the structures are remnants of rings of specialised tissue that surround the orifices of the tricuspid and mitral valves, while a third ring has been described encircling the aortic root. The function of these specialised areas is as yet unknown, although it has been shown that ablation of sites around the tricuspid and mitral valves is able to terminate atrial tachycardias. In this study, we used histology and immunohistochemical markers (Cx43, the major gap junction in the heart, and HCN4, the major isoform of the funny channel) in order to determine and clarify the location of the atrioventricular and aortic rings in rat (n=3) and guinea-pig hearts (n=3), and to establish the structural relationship of these tissues with the atrioventricular conduction axis. The animals were humanely killed (UK Home Office Schedule 1 method). Our study confirmed the presence of rings of histologically and immunohistochemically specialised myocytes surrounding the orifices of the mitral and tricuspid valves in both species (Figure 1). In the previous account, separate rings of specialised cells had been shown surrounding the orifices of the tricuspid and mitral valves (Anderson, 1972). Our present studies show that this interpretation was incorrect. Instead, each atrioventricular ring takes its origin from a dorsal extension from the atrioventricular conduction axis. The rightward extension runs around the vestibule of the tricuspid valve, whereas the leftward extension encircles the mitral valve. On returning towards the atrial septum, the tricuspid ring crosses over the penetrating part of the atrioventricular conduction axis, re-uniting with the mitral ring in the retroaortic node. Thus, there is a solitary ring around the atrioventricular canal, rather than separate rings around the tricuspid and mitral valves. Equally significantly, the atrioventricular conduction axis itself continues beyond the origin of the right and left bundle branches, this tract then forming an aortic ring that ascends towards the retroaortic node, but in the postnatal heart fails to make contact with the nodal structure. Our findings suggest that these areas of specialised tissues must subserve some function, since they are extensive in terms of the area they occupy.