In an earlier, rather extensive, publication in this journal on the "Functional Pathology of Hyperparathyroidism " (1) no mention was made of magnesium metabolism. Data have now been collected permitting a contribution to this phase of the subject. The methods of study were the same as described in the previous paper (1). Magnesium determinations were made by the procedure outlined by Briggs (2).The pharmacological relationship of calcium and magnesium ions and therefore the necessity for optimum concentrations of both in body fluids suggest a close association of the mechanisms regulating their level in the blood. The fact that magnesium phosphate has solubility properties similar to calcium phosphate and the fairly constant amount of magnesium in bone hint at a specific biological relationship of their compounds.It would seem that the parathyroid hormone might have at least an indirect influence on magnesium metabolism. The immediate effect of parathyroid hormone on serum magnesium has been studied in dogs by Scholtz (3) and by Greenburg and Mackey (4). Their investigations showed a slight increase in serum magnesium occurring early and before the rise in calcium. By the time calcium had reached maximum values the serum magnesium had fallen towards normal. Our experiments indicated that more prolonged administration of parathormone to dogs resulted in no striking alterations in serum magnesium (Table I). This was also true with rabbits in which parathormone is not very effective in raising the serum calcium.