The distribution of catecholamine of substantia nigra was observed under fluoromicroscopy by Falck-Hillarp method for catecholamine detection. Several mammals (monkey, goat, dog, cat, rabbit, guinea pig, rat and mouse) were used. Substantia nigra of all mammals used consisted of two regions, zona compacta and zona reticularis. Nerve cells and fibers with strong catecholamine-specific fluorescence were observed in zona compacta of all mammals used, but in zona reticularis nerve cells did not contain catecholamine and nerve fibers contained a little catecholamine. Nerve cells which consisted zona compacta of monkey, goat, dog and cat were large and sparsely distributed, while those of rabbit, guinea pig, rat and mouse were small and densely distributed. The strength of fluorescence of the latter animals were stronger than that of the former animals. Thus, the absence of significant differences among animals of distribution and cell shape of nerve cells of subatantia nigra suggest that this region has central role in the function of catecholamine, especially dopaminergic neurons of the brain.