The concentration of free amino acids was determined with an amino acid analyzer in the basal and peak secretion fractions of gastric juice obtained from 20 duodenal ulcer patients. 20 amino acids were identified in both fractions, with 5 amino acids, i.e. histidine, glycine, phenylalanine, leucine, and glutamic acid constituting over 50 per cent of the total amino acid concentration. The total concentration of free amino acids in gastric juice, similar in both fractions was lower than in blood serum. The total output of free amino acids was significantly higher in the peak fraction as compared with the basal fraction. The analysis of the results obtained suggests that besides the direct contribution of blood plasma amino acid pool there are two other sources of gastric juice amino acids: secretion from the cells of gastric mucosa and intragastric hydrolysis of proteins and peptides present in gastric juice by concomitantly secreted gastric proteases. No correlation was found between the values of output and concentration of gastric juice amino acids and the correspondent values obtained for hydrogen ion secretion.