Principal glycoproteins, uronic acid and amino acids were determined in canine gastric secretion, obtained from isolated canine stomachs perfused extracorporeally with homologous blood. Gastric secretion was collected during intraarterial infusion of pentagastrin alone for 3 h, and of secretin + pentagastrin for the next 3 h. Secretin produced a significant increase in the output of D-galactose, D-glucosamine, L-fucose, D-glucose, total sugars, total sulfate and uronic acid. Secretin had no significant effect on the output of most of amino acids of gastric secretion.