The paucity of data concerning the magnesium needs of man at different periods of his development has stimulated further study of magnesium retentions in children. The first report1 from this laboratory, which included a review of the literature, was concerned primarily with a series of single ten day balance studies with individual children. Calcium and phosphorus retentions also were studied in order to determine if there are any interdependent relations among these three metabolites.The present report represents part of a nine month study of the growth, osseous development and mineral metabolism of 3 preschool boys aged 4 years 10\m=1/2\months, 4 years 9 months and 3 years 3 months, respectively, at the beginning of the study and considers the magnesium retentions at three levels of ingestion, any one of which might occur in a seemingly well selected diet. Data pertaining to the calcium, phosphorus and nitrogen retentions and to the physical development of the children, as well as to the general plan of the investigation, have already been reported in The American Journal of Diseases of Children.2With the possible exception of magnesium at the lowest level of ingestion, the diets, which were constant from period to period, were believed to be adequate throughout the study. The modifications in the magnesium intakes were made chiefly by changing from a whole grain breakfast cereal and commercial whole wheat bread, supplying from 11.3 to 12.6 per cent of the total calories (period 1, table), to a refined breakfast cereal and white bread in one series of tests (period 2) and by decreasing the amount of egg and meat in a second series with refined breakfast cereal and white bread (period 3). In the third period 12.3 to 14.0 per cent of the total calories was supplied by cereal foods. Only distilled water was used for cooking and drinking ; thus the mag¬ nesium ingested came from food alone.