The distribution of catecholamine in corpus striatum, especially in caput nuclei caudati, was investigated by Falck-Hillarp method using several mammals (monkey, goat, dog, cat, rabbit, guinea pig, rat and mouse). The fluorescence of catecholamine was not observed in large and small nerve cells and bundles of myelinated nerve fibers in this nucleus of all animals used, but diffuse fluorescence was shown in neuropil (which consisted of myelinated and non-myelinated nerve fibers, dendrites of nerve cells and processes of glia cells) between these cell bodies. The ultrastructure of caput nuclei caudati of dog was also observed with electron microscopy. Intracellular organella were highly developed in large nerve cells, but not in small cells in this nucleus. Many synapses were observed in neuropil and these presynaptic fibers ended to postsynaptic fibers without large terminal boutons. These synapses were type 1 and 2 described by Gray. These results suggest that the sites where strong fluorescence showed in caput nuclei caudati of all mammals corresponded to the sites where many synapses existed, and that catecholamine was involved in nerve endings in these synapses.