It is well established that the atrioventricular (AV) conduction axis is the so-called “specialised” component of the junctions between the atrial and ventricular myocardial tissues, and that the bundle of His is the only muscular structure that, in the normal heart, crosses the plane of AV electrical insulation. It has remained controversial, however, as to whether other parts of the atrial vestibules are histologically specialised. Kent, in 1893, proposed that multiple muscular bridges cross from atrium to ventricle as the substrate for normal AV conduction. It is certainly the case that, in human hearts, remnants of histologically specialised myocardium are to be found on the atrial side of the insulating plane, albeit without making muscular connections with the ventricular myocardium, but the full extent of these so-called AV ring tissues has still to be established. In the present study, using histology and immunoconfocal microscopy, we have investigated the specialised components of the AV junction in the hearts of four rats and guinea-pigs. The animals were humanely killed, and the hearts rapidly removed and frozen in liquid nitrogen. 20 µm serial sections were then cut through the entirety of the AV junctions. In both species, extensive areas of histologically discrete myocardium are found in the vestibules of the tricuspid and mitral valves. The areas are more extensive in guinea-pig than rat, and show the immunohistochemical characteristics of pacemaker tissue. The best developed part of these specialised tissues, apart from the AV conduction axis itself, is found in the atrial septum immediately posterior to the aortic root, an area previously described as the retroaortic knot (RAK). Immunofluorescence and confocal microscopy revealed that, as with the AV conduction axis of the heart, this RAK tissue is positive for the major isoform of the funny channel, HCN4, and negative for the major gap junction channel, Cx43. In addition, the RAK tissue, like other cardiac myocytes, expresses caveolin3, a membrane bound protein. We used the expression of caveolin3 to measure the diameter of RAK cells. The RAK cells are significantly larger than AV node cells, but significantly smaller than atrial and ventricular cells (10.34 ± 0.99, 6.76 ± 0.38, 14.45 ± 0.61 and 18.92 ± 0.58 µm, respectively). In both species, the RAK is in continuity with the rings of specialised myocytes that encircle the orifices of the tricuspid and mitral valves. We conclude that the ring tissues, including the RAK, expressing HCN4, may be capable of initiating spontaneous pacemaker activity, and could be a source of arrhythmias.