1. l-Aspartic acid labeled with N15 was fed to one human adult and six infants, and the total N and N15 were determined in the urine from time to time.
2. The N15 concentration (or isotopic ratio) of urinary N reached its maximum in the adult about 2 hours and in the infants about 4 hours after feeding, then fell off logarithmically.
3. Assuming that the N of aspartic acid readily entered into equilibrium with other N compounds in the pool, the rate of turnover of the N pool was calculated from the rate of fall of the isotopic ratio of urinary N. This rate of turnover of N was about 4 per cent per hour in the adult and 6 to 12 per cent per hour in the infants.
4. The rate of protein synthesis calculated from the rate of turnover of N was 10 mg. N per kilo per hour in the adult and 18 to 27 mg. N per kilo per hour in the infants, with one exception which showed a higher rate of 52 mg. N per kilo per hour. The size of the metabolic pool of N per kilo in non-growing infants was about the same as that in the adult (0.4 to 0.5 gm.) but it was somewhat larger in growing infants (0.5 to 0.8 gm.).