The sodium and water balance of rats was varied, and changes in the number and distribution of specific atrial granules (ASG) were observed electron microscopically. The number of ASG in atrial muscle cells increased in rats without salt intake for 7-8 days. On the contrary, the number of ASG began to decrease 10 days after rats were given 2-2.5% sodium chloride solution ad libitum instead of water. After 30 days, the number of ASG decreased markedly in the central sarcoplasmic core, in the sarcoplasmic layer and under the cell membrane. Most atrial muscle cells in rats 6 days after hypophysectomy contained less ASG than those of control rats, and 30 days after the hypophysectomy, only a small number of ASG were observed in the central sarcoplasmic core, in the sarcoplasmic layer and under the cell mumbrane. The number of ASG increased the first 5 days after adrenalectomy, but did not change after 7 days. The number of ASG increased a little 4 weeks after birth in spontaneous hypertensive rats (SHR). ASG increased up to 20 weeks after birth of SHR, concomitantly with an increase in the blood pressure. To study the mode of secretion of ASG in detail, Ca-ionophore A23187 was used. In ionophore-treated cells, exocytotic figures were clearly demonstrated.