Specific granules in the atrial muscle cells were examined in normal and treated rats with electron microscopy. By the Epon-embedded section, the specific granules existed in sarcoplasm along the nucleus and in sarcoplasm between myofibrils or beneath the cell membrane, and distributed as the granules with the electron dense core. By the freeze-etching techniques, there were many membrane-associated particles on PF-surfaces of the limiting membrane of the granules. By the administration of acetylcholine the shape and the distribution of the granules didn't change. But after the administration of atropine the granules increased, and after the administration of eserine these decreased in number without showing morphological changes. By the administration of monocrotaline, the specific granules of the right atrial cells increased in perinuclear after 13 days, and increased in all areas of the atrial muscle cells according to many days and much dose of the administration. On the basis of these experimental results, the function of these granules was discussed.