A 21-day metabolic balance study was designed to test the hypothesis that dietary zinc level affects protein utilization in growing children. Subjects' mean age was 8 years 5 months, mean weight was 29.5 kg, and mean height was 132.1 cm. The subjects were divided on the basis of weight into four treatment groups following a 2x2 factorial experimental design. After an eight day adjustment period in which all subjects consumed a mixed diet known to contain 28.8 g protein, 5.5 mg zinc, and 2,000 Kcal, the four treatments were administered for 12 days. One group was supplemented with the essential amino acids found by analysis to be most limiting with respect to whole egg protein. Threonine, valine, methionine, isoleucine, and lysine were added in amounts to reach a chemical score of 80 based on whole egg protein. The sources and amounts of nitrogen were: food, 4.18 and supplements, 0.14 g/day. One group received these same amino acids plus a zinc supplement of 5.0 mg/day. One group received an isonitrogenous amount of ammonium citrate matching the nitrogen contained in the amino acids supplement. The last group received the isonitrogenous amount of ammonium citrate plus 5.0 mg zinc. All supplements were given daily and were incorporated into meals. All urinary and fecal excretions were collected and analyzed for nitrogen and zinc. Nitrogen analysis was accomplished using a modified Kjeldahl-Gunning-Arnold method and zinc was determined spectrophotometrically on wet-ashed samples. Nitrogen retentions for the four groups were: amino acids plus zinc, 1.00; amino acids, 1.01; ammonium citrate, 0.90; and ammonium citrate plus zinc, 0.81 g/day. Analysis of variance on the balance data from the four groups showed no effect of zinc, added essential amino acids or the interaction of the two on the nitrogen balance of the subjects, although the trends of retention favored amino acids. There was a significant effect (p<.01) of added zinc on zinc balance, however. It was concluded that zinc did not affect nitrogen utilization in the growing child, but that zinc balance is affected by zinc quantity in the diet. The present Recommended Dietary Allowance of 10 mg zinc/day for this age group is the minimum necessary to maintain positive zinc balance.Ph. D.