ExtractSignificant amino acid loss through the stool, analogous to impeded reabsorption of water, fat, and electrolytes, was suspected in infantile diarrhea. The stools of six patients, who had acute diarrhea and received no protein by mouth, were collected and analyzed for amino acids, nitrogen, and protein. Similar determinations were made on five normal controls. In addition, stool collections were made from one patient during a severe protracted diarrhea which necessitated parenteral alimentation, during a subsequent isolated acute episode of diarrhea, and after complete recovery. The results of the analyses showed that free amino acids in the stool are elevated up to 10-fold during diarrhea, as compared with periods of remission and with normal controls. Protein and total nitrogen excretion is also elevated in diarrhea, although to a lesser degree (4-and 2.5-fold, respectively). It is postulated that protracted diarrhea is associated with impaired reabsorption of endogenous nitrogen, which results in a vicious cycle of protein depletion that ultimately leads to an intractable stage. The validity of this assumption is confirmed by the success achieved by parenteral amino acid therapy for protracted diarrhea.