The function of atrial specific granules has been obscure in spite of many morphological studies. However, the relationship between these granules and sodium and water metabolism has been suggested, and it has been confirmed that heart atrium extract has a diuretic action. An atrial natriuretic peptide (ANP) has been purified from the human atrium. The author investigated the kinetics of specific granules and discussed the relationship between these granules and ANP. A detailed study on the mechanism of secretion of these granules was also performed. The number of atrial specific granules in rats fed at 5℃ for 50 days tended to decrease, and number in rats fed at 33℃ for 50 days increased. The number of granules in rats fed with 0.5-1% sodium chloride solution decreased after 45 days. An increase in the number of granules was observed after rats were fed without water 11 days. This increase in granules was observed in the sarcoplasm between myofibrils, and just beneath the plasma membrane as well as in the sarcoplasm at the periphery the nucleus. The continuous administration of diurtics (Lunetron) induced a decrease in granules at the periphery of the nucleus, while antidiuretics (Pitrssin) caused an increase in granules around the nucleus, between myofibrils and beneath the plasma membrane. Type Ⅱ granules were observed near Golgi apparatus, and their contents were similar to the materials seen in Golgi apparatus. From morpholgical studies, it was concluded that these type Ⅱ granules matured to type I granules, and that they were transferred to the cell periphery and their contents were released by exocytosis.