The porcine sinoatrial node in an isolated right atrium preparation is characterized by unifocal impulse generation. It has a rather elongated shape and the larger part of its volume is taken up by collagen and fibroblasts. The impulse appears to emerge from a site where the percentage of myofilaments is relatively low. The impulse is propagated faster towards the crista terminalis than to the interatrial septum with preference for the oblique-upward direction. A very large zone of cells with low excitability is located at the interatrial septal side of the node.