The specialized tissues of the conduction system, comprising the S-A and A-V nodes and the Purkinje fiber, of the cow heart were observed with the electron microscope. Sections from each region were prepared by strict light microscopic identification. At the submicroscopic level, all these tissues have been revealed to show some characteristic features different from the ordinary myocardium. In any region of the conduction system the specialized fibers are composed of cells which have distinct plasma membranes of their own, though the cells vary in size, shape, and arrangement from one region to another. In the Purkinje fiber very large cells are arranged in several rows ; some of the cells are totally surrounded by other Purkinje cells. In the A-V and S-A nodes, though the cells, much smaller in size, are contiguous poly-hedrally with one another, they are never arranged in so many rows as in the Purkinje fiber. At the junction between the A-V node and the Purkinje fiber, the nodal cells are contiguous with the Purkinje cells. The cell junctions throughout the conduction system are studded with desmosomes. The myofibrils are generally sparse and are distributed preferentially in the periphery in the Purkinje cells, but rather diffusely in the nodal cells. The sarcosomes are smaller in size and more frequently elongated than in ordinary myocardial cells. The sarcoplasmic reticulum appears to be poorly developed, mostly in smooth-surfaced profiles. The pinocytotic vesicles are especially conspicuous in the S-A nodal cells. Multinuclear cells are frequently encountered throughout the conduction system. The Golgi apparatus, dense bodies and pigment bodies are cccasionally seen in the perinuclear sarcoplasm. Small dense granules which are considered to represent deposits of glycogen may occur in the sarcoplasm in lead hydroxide stained preparations. In the interstitium of the conduction system, many nerve fibers, mostly unmyelinated. exist close to the specialized fibers ; some of the axons contain mitochondria and many synaptic vesicles and rarely appear to have their endings on the nodal muscle cells. In the S-A node, smooth muscle fibers may be seen. The nodal artery in the S-A region has a well-developed muscular layer in the tunica intima. The internal elastic lamina appears not to form a complete membrane but to be cribriform. The physiological significance of these findings is discussed briefly.