30-cm segments of jejunum have been perfused with amino acid solutions in human subjects. The changes in amino acid composition of the perfused fluid have been studied in normal subjects, and in patients with the coeliac syndrome and chronic pancreatitis. The amino acid composition of fasting succus entericus in various conditions, and its relationship to plasma amino acids have been investigated. Evidence is presented to show the presence of an efficient mechanism for amino acid homeostasis in the small bowel of man. Such homeostasis is brought about mainly by the release of endogenous amino acids from the pancreas and small intestinal mucosa.