It is evident that fishes regulate their serum calcium efficiently but that endocrine systems involved may be different from those in tetrapods. A functional parathyroid gland has not yet been demonstrated in fishes. The majority of evidence indicates that calcitonin has little or no effect on fish calcium regulation. Instead, the corpuscles of Stannius and the pituitary gland are necessary for maintaining fish serum calcium levels. In the killifish, Fundulus heteroclitus, the removal of the corpuscles produces hypercalcemia in sea water but not in artificial sea water deficient in calcium. Transplants of the corpuscles or the administration of corpuscle homogenate corrects the increase in calcium. On the other hand, hypophysectomy elicits hypocalcemia under calcium deficient conditions but not in calcium rich sea water. Replacement therapy with pituitary homogenate or hypophysial transplant prevents the fall in calcium. It is postulated that the hypocalcemic corpuscles of Stannius and the hypercalcemic pituitary gland enable the euryhaline killifish to regulate its serum calcium levels in high calcium sea water and low calcium fresh water, respectively.