The effect of calcitonin (CT) on serum glucose and insulin secretion was investigated in rats. The subcutaneous administration of CT (80 MRC mU/100 g body weight) produced a significant increase in serum glucose, while it did not significantly alter serum insulin in fed rats. In addition, a marked elevation of insulin secretion after a single intraperitoneal administration of glucose (0.1 g/100 g) in fasted rats was not significantly altered by the treatment with CT. When both CT and somatostatin (250μg/100 g) were subcutaneously administered simultaneously, the serum glucose level increased significantly compared with that of rats given somatostatin alone. The progressive increase in serum glucose caused by CT was significantly inhibited by the subcutaneous administration of insulin (0.1 U/100 g). These results indicate that CT increases serum glucose concentration independently of insulin secretion in rats.