In isolated preparations of the rabbit sinus node, we have investigated the fine structure of the tissue at many sites which had been electro-physiologically identified by means of microelectrode recordings. For each of these sites we quantified the myofilament density of the cells, since this appeared to be a useful parameter for characterizing cell types in the sinus node region; and because myofilaments were present both in isolated form and organized in myofibrils, the degree of organization was also measured in a semi-quantitative fashion. The electrical activity of cells at a given site was characterized by the activation moment relative to the cardiac cycle and furthermore by the rate of diastolic depolarization and the maximum rate of rise of the action potential. From the centre of the node toward the periphery a very gradual increase in myofilament density was observed in all directions. It was found that the rate of diastolic depolarization, which feature is generally accepted as being basic to the automaticity of the sinus node, was inversely related to the volume percentage of myofilaments. This means that a relation exists between the pacemaker action and the cell type. The anatomically less developed cells, i.e. the cells with the lowest density of organelles, which are located in the central portion of the node, are the most specialized pacemakers. No clear relation was found between the myofilament density and the rate of rise of the action potential. In the direction of the crista terminalis we observed an increase in the rate of rise and an increase in conduction velocity concomitant with the increase in myofilament density. Toward the interatrial septum, however, the increase in myofilament density was not accompanied by an increase in rate of rise; in this direction the impulse conduction was blocked. A correlation between cell type and impulse conduction could thus not be established.