The sinus node (SN) is located in the area between the superior caval vein and the right atrium.1 It is the first component of the cardiac conduction system where the initiation and propagation of the electrical activity in the heart begins. Transverse tubules (t-tubules) are invaginations in the cell membrane of mammalian ventricular myocytes. L-type Ca2+ channels are located on these membranes, in close proximity to the sarcoplasmic reticulum (SR), and trigger, Ca2+-induced Ca2+ release.2 T-tubules are not present in the atrial and nodal myocytes of small animals such as rats 3, however, atrial myocytes of larger animals such as sheep do have t-tubules.4 This raises the question: are these structures present in the primary pacemaker cells? The aims of this study were to investigate (1) if t-tubules are present in rat and/or human SN myocytes and (2) is ryanodine receptor (RYR2) organization similar to that observed in ventricular myocytes. Ventricular and nodal myocytes were isolated by enzymatic digestion from male Wistar rats. All procedures were in accordance with the Animal (Scientific Procedures) Act 1982. Immunocytochemistry and confocal microscopy was performed on isolated myocytes (n=29 nodal and n=12 ventricular cells; from n=8 rats). Cells were labelled with wheat germ agglutinin (WGA) as cells membrane and its invagination marker and an RYR2 antibody as specific marker for RYR2. Immunohistochemistry and confocal microscopy was also performed on tissue sections from frozen human SN and its surrounding atrial muscle (n=3 healthy specimens). Specimens were stored under the Human Tissue Act 2004. Tissue sections were labelled with: (1) WGA, (2) RYR2, (3) alpha-actinin (a marker of Z-lines of myofibrils), (4) HCN4 (responsible for funny current that contributes to the pacemaker potential), (5) Cx43 (a major gap functional channel responsible for electrical activity between working myocardial cells) as a positive marker and negative marker of the SN respectively and (6) Caevolin3 (a cardiac cell membrane marker). Our results showed that rat isolated nodal cells lack t-tubules (by the absence of a striated pattern of WGA labeling) but RYR2 expression in the SN was striated. In ventricular cells both WGA and RYR2 expression[LM3] pattern was striated and signal pattern was identical with clear t-tubular labelling. In tissue sections, we observed that some human atrial cells contained t-tubules. The SN cells did not contain t-tubules and the striated pattern of RYR2 expression was co-localised with alpha-actinin. HCN4, Cx43 and Caevolin3 expression was the same as previously reported by Chandler et al.,5. HCN4 was present and Cx43 was absent in the SN and Caevolin3 was expressed in all atrial and nodal myocytes. We conclude that a striated pattern of RYR2 expression and its co-localisation with alpha-actinin is important for efficient Ca2+ signalling in these specialised cells.