Distribution of Purkinje fibers, peripheral branches of impulse conducting system, in subendocardial tissue and in the myocardial tissue of various mammalian hearts were studied by histological methods (Otsuka & Hara method, PAS staining method, Masson-Goldner method and hematoxylin-eosin method). Purkinje fibers were distributed in the subendocardial tissues of cow, pig, goat, monkey and dog. And these distribution patterns were varied among animals. But, Otsuka & Hara method could not clarify the distribution of Purkinje fibers in subendocardial tissues in cat, rabbit, guinea pig, rat and mouse. In human's heart, Purkinje fibers were not observed because the specimen was not fresh. The observation of Purkinje fibers in myocardial tissue revealed that these fibers penetrated into heart muscle layer deeply and changed into common heart muscle fibers in cow, pig, and goat and that in other animals these fibers were not observed in myocardial tissues but changed into common muscle fibers in subendocardial tissues. These results indicating the species difference of distribution of Purkinje fibers suggest that impulse conductions also differ among animals in the physiological aspects.