The description ofthe body composition of a 9-yr-old reference boy has been presented in detail(1). We used the 50th percentile values for weight and height from the data of the National Center for Health Statistics (NCHS)(2) and corrected other data to agree with this height and weight. Data from the literature concerning total body water (TBW), total body potassium(TBK), and total body calcium(TBCa) were also used. TBW was determined from volume of distribution of deuterium and TBK from total body 40K counting. Because potassium is nearly absent in nonaqueous compartments of the body, we estimated the size of the cellular and extracellular water compartments from the known quantities of TBW and TBK and knowledge of the concentration of potassium in extracellular and cellular wa-ter (4 and 150 mEq/kg, respectively). The quantity of nonosseous minerals was ob-tained by multiplying the quantity of cellular water by its known concentration of minerals and the quantity of extracellu! ar water by its known concentration of minerals. Body content of osseous minerals (ie, minerals of hard tissue) was calculated from the body content of calcium and data in the literature demonstrating that calcium accounts for 34% by weight of bone cortex. An approximation of TBCa was obtained from the studies of Chris-tiansen et a!.(3), based on two-dimensional photon absorptiometry. We calculated protein content of the body(nitrogen x 6.25) on the assumption that the ratio of nitrogen to potassium is the same in the 9-yr-old as in the adult. We assumed that in the 9-yr-old, as in the adult, glycogen accounts for 0.5% of body weight.