1. [15N]Glycine was infused into fed, fasted or acidotic humans. 2. In all metabolic states there was considerable transfer of labelled nitrogen to urea and ammonia, but alanine and glutamate did not become enriched. 3. The findings show that free exchange of nitrogen between all amino acids does not take place. Glycine, serine, threonine, lysine and histidine cannot be considered part of the alpha-amino-nitrogen pool as classically conceived, although they form up to one-half of that pool.