Specific granules of heart atrial muscle obtained from mammals (mouse, rat, rabbit, dog, pig, ox and monkey), aves (chicken), reptiles (Geoclemmys), amphibia (Rana), pisces (carp, goldfish and shark) and cyclostomata (Entosphenus) were investigated by electron microscope and the following results were obtained. The mean diameter of the specific granules was large in rat (315 nm) and mouse (283 nm), medium in rabbit (260 nm) and pig (210 nm) and small in dog and ox (170 nm) and monkey (175 nm) among mammals. The core of these granules was electron-translucent in rat and mouse and electron-dense in rabbit. The atria of rat and mouse had a large number of granules while that of monkey showed a fairly large number of granules. The dog showed a small number of granules and the granules of ungulate (pig and ox) were sparse. Granules in vertebrates other than mammals were relatively smaller than in mammals. The mean diameter was 144 nm in chicken, 175 nm in Geoclemmys, 150 nm in Rana and carp and 168 nm in Entosphenus. The atria of Geoclemmys and Rana had a large number of granules while that of Entosphenus also had a fairly large number of granules. Shark only showed a small number of granules and the granules of chicken, carp and goldfish were sparse. Electron microscopic study of specific granules in rat heart atrial muscle showed that there were two types of granules. One (Type I) had a homogenous and electron-dense core and the other (Type II) was electron-translucent and consisted of small granular materials. Both were enclosed by a limiting membrane. Type II were distributed near the Golgi apparatus present at the perinuclear area of the muscle cell while Type I were widely distributed in sarcoplasm at the perinuclear area, between myofibrils and beneath the cell membrane of the cell. From the results, the process of the formation, maturation and release of specific granules of heart atrial muscle was discussed.