The patterns of N15-excretion, as shown by isotope concentration-time curves for ammonia, hippuric acid and urea, are different in the first few hours after oral and intravenous administration of N15-labeled glycine. After 6–8 hours, the curves are substantially the same. Therefore, data obtained in oral and in intravenous experiments should not be treated indiscriminately from the beginning. The ratio of the isotope concentration of glycine (calculated from that of hippuric acid) to that of ammonia should, on the basis of certain assumptions, theoretically begin with a high value (i.e. 20) and fall eventually to 1 when transamination equilibrium is reached. In an oral experiment the ratio found is 8.5 at 1 hour, 6.1 at 2 hours and 2.6 at 14 hours. It may be concluded that the transfer of NH2 from glycine to the other amino acids is not so rapid, compared with rates of other metabolic reactions of the amino acids, as to justify the assumption of a single pool for all amino acids from the beginning. The implications of this finding with reference to interpretation of N15-excretion data are discussed, and some sample calculations with data in the glycine experiments are given in a subsequent paper. Submitted on September 15, 1958