In the investigation of various problems of calcium and phosphorus metabolism, it has been found that great changes in the volume and direction of the calcium and phosphorus stream may occur without any notable difference in the serum levels of these elements. In hyperthyroidism the calcium excretion in the urine is relatively huge and there are large negative balances of calcium and phosphorus, yet the serum calcium and serum phosphorus' remain within normal limits (1). Moreover, there is no appreciable change in the serum levels of healthy people when, by changing the calcium intake, a positive balance is shifted to a negative one or vice versa.Yet the actual and relative amounts of calcium and phosphorus in the serum is of the utmost importance in certain abnormal conditions. MacCallum and Voegtlin (2) described the low blood and tissue calcium associated with tetany after removal of the parathyroids.Greenwald (3) later demonstrated the associated rise in serum phosphorus and since that time the high phosphorus, low calcium of the serum in many types of tetany has come to be recognized. Marriot and Howland (4), DeWesselow (5) and others have found a similar relationship of the serum values in terminal nephritis. On injection of large amounts of inorganic phosphate into dogs Binger (6) produced tetany with a low serum calcium and high serum phosphorus. Similar results have been found to follow injection or ingestion of very large quantities of phosphate by numerous investigators (7,8,9). The 1 Throughout this paper the term "serum phosphorus" is used in referring to serum inorganic phosphorus.