This study is aimed at determining the amounts of each essential amino acid required for adequate protein nutrition by children two years old. These data are not available in the literature. Two year old children will be given diets partially made of cow's milk protein, supplemented with pure amino acid mixtures. The level of each essential amino acid under investigation will be varied to determine the minimum intake which will support physical and biochemical normality. Careful clinical surveillance, oxygen consumption, nitrogen balance and variations in blood and urine nitrogenous components (amino acids, creatinine, urea, protein electrophoretic fractions will be measured in a systematic way. The children will be located in the INCAP Unit for Metabolic Studies, but will not be restricted in their normal activities except during the nitrogen balance periods. The data resulting from this research will permit to formulate a pre-school age specific protein pattern to assess protein and amino acid nutrition at this age more exactly.