Fine structure of the normal left and right bundle branches was studied with the electron microscope in one human and two canine hearts and correlated with light microscopical observations from 10 human and 10 canine hearts. There was no significant species difference. Though the left bundle branch was comprised of typical Purkinje cells, there were numerous interspersed cells with the appearance of ordinary working myocardium. Geometric arrangement of the cells in the proximal portion of the left bundle branch was in one thin sheet divided by numerous longitudinally oriented collagen partitions, resembling a direct continuation of the histological geometry of the His bundle. Intercellular junctions within the left bundle branch were through intercalated discs which contained many long profiles of gap junction, in contrast with working myocardium, where gap junctions are much smaller. The right bundle branch was a thin cylinder rather than a sheet and did not contain any apparent partitioning pattern. Typical Purkinje cells are comparatively few in the right bundle branch and cells with numerous myofibrils (appearing as working myocardium in light micrographs) predominated. Junctions between these cells contained fewer gap junctions than those in the left bundle and very long profiles were less numerous. Widened Z bands were conspicuous in the right bundle branch cells of one dog. Some of the physiological implications of these fine structural features are discussed relative to electrical properties of the two bundle branches.