The effects of calcitonin (CT) and epinephrine on serum glucose concentration were investigated in fed rats. The administration of CT (80 MRC mU/100 g body weight) to adrenalectomized rats caused a significant increase in serum glucose. The simultaneous administration of CT (40 and 80 MRC mU/100 g) and epinephrine (7.5 and 15 μg/100 g) produced a clear increase in comparison with that caused by epinephrine alone. On the other hand, CT did not change the glucose tolerance, while epinephrine markedly lowered it. The present results suggest that the hyperglycemic effect caused by CT is not mediated through the action of epinephrine from adrenal glands in rats.