The primary pacemaker, i.e. the group of pacemaker cells discharging the sinoatrial node comprises less than 1000 cells in the guinea-pig and about 5000 cells in the rabbit. These primary pacemaker cells are described as 'central nodal' cells in light microscopy and as 'typical nodal' cells in electron microscopy. The action potential of the leading cells has a higher upstroke velocity in the guinea-pig than in the rabbit (6.2 v. 1.9 V/s). Gap junctions have been observed even in the very center of the node in both species. A zone of double-component action potentials at the septal margin of the node was observed in the rabbit, but not in the guinea-pig. Evidence is presented for abrupt transitions in electrophysiological as well as in ultrastructural characteristics in the guinea-pig sinoatrial node. The differences in intrinsic cycle length between both species but also between individuals of the same species are discussed.