Synthetic porcine calcitonin (\g=a\-calcitonin) and its methionine-sulphoxide derivative (\g=b\-calcitonin) were given by intravenous infusion to conscious male rats. \g=a\-Calcitonininactivated by performic acid oxidation was used as a control.Microgram doses of \g=a\-calcitonin produced a dose-dependent decrease in the renal excretion of magnesium. The effect was not due to a secondary release of parathyroid hormone since it was also seen in parathyroidectomized animals.A marked increase in the renal excretion of inorganic phosphate, sodium and potassium preceded the change in magnesium excretion in parathyroidectomized rats. It is concluded that the phosphaturia and natriuresis previously described after administration of extracted calcitonin preparations are true effects of the hormone.The effect of \g=b\-calcitonin was indistinguishable from that of \g=a\-calcitonin.