AbstractAtrioventricular nodal tissue obtained from a two‐week‐old mouse heart was fixed in osmium tetroxide, embedded in Epon 812, and serially sectioned for electron microscopy. From the study of electron micrographic montages of the first 620 serial sections of the posterior‐inferior (tail) portion of the node, it was determined that many of the nodal cells are spindle‐shaped and each has a single, centrally‐placed, ellipsoidal nucleus. Other cells exhibit irregular surfaces with protrusions of variable dimensions and some are flattened. Most of the flattened cells are located in the peripheral borders of the posterior and lateral portions; irregularly‐shaped cells occupy deeper positions in the tail of the node. Two cells, which are different from all others, reside in the peripheral border of the medial portion. They are contiguous with one another and globular in shape, each containing a single, centrally‐placed, spherical nucleus. These two cells are sparsely endowed with myofilaments but contain many mitochondria. They may be pacemaker cells. Intercellular relationships among nodal cells exhibit a wide variety of complicated contact areas in addition to some rather simple contiguities of the lateral aspects of adjacent cells. No typical intercalated discs are present; however, junctional specializations between interdigitating protrusions of contiguous nodal cells do occur. Intimate contact between vesiculated nerve processes and nodal cells varies in extent from a profuse innervation in the posterior and lateral portions of the tail of the node to lack of contact with many of the cells in the medial and anterior portions. This pattern of innervation, in conjunction with the absence of maculae occludentes between cells of the posterior and lateral portions of the tail of the node but present between cells of the other portions, suggests that the medial and anterior areas are conductive and the posterior and lateral areas a