The monoaminergic innervation of the mammal (ox, monkey, dog, cat, rabbit, guinea pig, rat and mouse) heart was studied by a histochemical fluorescence technique. The number and distribution of monoaminergic nerve terminal was greater in the atria than in the ventricles of the heart of mammals. Fluorescence histochemical studies of the mammal heart conducting system have shown the density of monoaminergic nerve terminals to be highest in the sino-atrial node, followed in order by atrio-ventricular node, atrio-ventricular bundle and false tendons. The plexus was so extensive that the majority of sino-atrial nodal cells appeared to be innervated. The innervation to the Purkinje cell in the false tendons from both the right and left ventricle of mammals except for ox was of low density and a large number of cells seemed to have no contact at all with the monoaminergic terminals. In the cases of 6hrs after reserpine administration, fluorescent nerve fibers had disappeared from all mammal heart. In cases in which L-DOPA oder nialamide was administered 3-5hrs before killing, fluorescent nerve fibers in mammal heart gave an intensified fluroescence together with the increased number of fluorescent fine varicose nerve fibers. After right and left stellate ganglionectomy, histochemical determinations showed a decrease in monoaminergic nerve content both right and left atria, in both right and left ventricle, and heart conduction system.