The reports on the amount of the three metabolites—calcium, phosphorus and nitrogen—needed by children of a given age have varied, because, it is believed, there were differences in the growth pattern of the children studied, in the degree of saturation of the tissues at the time of the study and in the availability of the metabolites in the materials fed. Furthermore, the retention of calcium and nitrogen is influenced not only by the amount needed but by whether an adequate amount of phosphorus is supplied by the diet. In many of the studies only calcium balances were reported; comparisons, therefore, may not be valid. The problem is further confused by the fact that many of the studies have been of short duration. Continuous balance studies both with children and with adults have indicated that there is considerable variation from period to period; and although this is due in part to