The effects of intraperitoneal calcium infusion and of oral administration of calcium were studied in thyroidectomized (TX) and thyroid—intact (TI) rats. In those animals administered calcium orally, the kidneys were removed 2 hr prior to calcium administration. 45Ca and 32P were administered 20 min prior to calcium infusion and five minutes after the oral calcium dose. In some cases salmon calcitonin (SCT) was infused with the calcium. The following results were noted: Plasma calcium values were significantly lower in all TI rats following calcium administration than in their TX counterparts. However, plasma calcium values rose in all animals as a result of calcium treatment. Plasma phosphate concentrations rose following calcium administration in TX rats, whereas they fell in rats with functional thyroids. The infusion of SCT with calcium to thyroidectomized rats partially prevented the rise in both plasma calcium and phosphate values. The presence of endogenous calcitonin increased the disappearance rate of 32P from plasma but it had no detectable effect on that of 45Ca. The relative specific activities of 45Ca in plasma were, therefore, higher in rats containing intact thyroids. However, because the change in rate of 32P disappearance from plasma of rats produced by calcitonin was of the same magnitude as the change in stable phosphate, plasma 32P SA was not changed by the hormone. This effect of calcitonin was also observed in nephrectomized animals. It is concluded that the endogenous secretion of calcitonin which results from increases in plasma calcium concentrations during infusion or following oral administration of this ion provides sufficient hormone to prevent the rise in plasma phosphate concentration which otherwise would occur. (Endocrinology92: 792, 1973)