The distribution of acid phosphatase and succinic dehydrogenase in the specialized muscle fibers constructing the heart conduction system (sinoatrial node, atrioventricular node, atrioventricular bundle and Purkinje fibers) and the ordinary heart muscle fibers of several mammals were studied histochemically with a light and electron microscopies. The activity of acid phosphatase was high in the specialized muscle fibers of dog and monkey, relatively high in these fibers of rabbit, guinea pig, rat and mouse and low in cow and pig. The activity in the ordinary heart muscle fibers was low in all mammals studied. Embryologic study of the enzyme in rat showed that equal activity was found at the late fetal stage in both heart muscle fibers and increased only in the specialized fibers 30-40 days after birth. Electron microscopic study on the distribution of the enzyme in rat's atrioventricular node showed that the reaction product was found on myofibrils and chromatin in both P cells and T cells which consisted the node. Succinic dehydrogenase activity was studied using mammals described above including cat. The activity in the specialized heart muscle fibers was negative or weakly positive throughout the developing stage of all mammals used, while its activity in the ordinary muscle fibers was already high 1-2 days after birth. From these results, the role of both enzymes in the heart conduction system was discussed.