Nitrogen balance studies were performed with 12 normal full-term infants, 8 boys and 4 girls, during the first 4½ to 6 months of life. Three high-protein feedings were employed, supplying approximately 20% of the calories as protein from cow milk. Growth of the infants is interpreted as within normal limits. The concentration of total protein in the serum was not greater than that recorded in previous studies employing lesser dietary intakes of protein. The mean concentration of urea nitrogen in the serum was 21.8 mg/100 ml between 1 and 6 months of life and somewhat greater during the first month of life. A close relation between mean intake of protein in various studies and mean concentration of urea nitrogen was demonstrated. Intakes of protein were generally between 4.5 and 6 gm/kg/day, and retentions of nitrogen were, with few exceptions, between 200 and 300 mg/kg/day in the first 2 months of life and between 130 and 200 mg/kg/day from 4 to 6 months of age. The major sources of error in calculation of body content of protein on the basis of metabolic balance studies are mentioned. Metabolic balance studies are believed unsuitable for prediction of body composition.