Succus entericus from jejunal Thiry-Vella fistulae in fasting conscious dogs was analyzed for amino acids before and after incubation for 15 and 30 min at 38 °C in vitro. Total amino acid concentration of succus entericus as collected was about twice that of blood plasma. After incubation, it was increased 76% above control at 15 min and 137% at 30 min. Because succus entericus does not exhibit proteinase activity, and no enzyme or substrate was added before incubation, the increase in amino acid concentration was probably due to hydrolysis of peptides by peptidases normally present in the juice. Volume of secretion and output of amino acids were greatly increased by intravenous administration of glucagon, but the amino acid pattern was quite different from blood plasma. The concentrations of aspartic and glutamic acids were 10- and 5-fold greater than in plasma, and tryptophan was usually undetectable