The impulse conducting system of the hearts of several mammals (monkey, cow, pig, dog, cat, rabbit, guinea pig, rat and mouse) were studied using light microscopy and electron microscopy after cholinesterase (ChE) reaction were done. Under light microscopic observation, ChE reaction were strongly positive in the specialized muscles constructing sinoatrial node, atrioventricular node and atrioventricular bundle of all animals used. But the reaction in Purkinje fibers varied among animals. Cholinergic nerve fibers, which were ChE reaction- positive, were distributed mostly in both nodes regions. By electron microscopic observation, ChE-positive materials were present on the myofilaments and sarcoplasmic reticulum of specialized muscle fibers in both nodes. Observation of nerve terminals distributed in both nodes after ChE reaction revealed that ChE-positive materials were present in the narrow space between cell membranes of nerve cells and vicinal Schwann cells. The reasons why the specialized muscle fibers of both nodes were positive in ChE reaction were not clear, but dense distribution of cholinergic nerve terminals at both nodes suggest that these regions are richly innervated by autonomic nerve fibers.