The effects upon islet hormone secretion of highly purified preparations of secretin and of pancreozymin-cholecystokinin and of a crude gastrin-containing extract of hog antrum have been studied in acutely operated dogs. All three preparations were shown to cause a striking increase in insulin concentration in the pancreaticoduodenal venous plasma after their rapid endoportal injection in anesthetized dogs. With each hormone preparation, the peak in insulin secretion occurred 1 minute after injection, and a rapid decline was observed immediately thereafter. Whereas secretin and gastrin failed to alter significantly the pancreaticoduodenal venous glucagon or arterial glucose concentration, pancreozymin caused a dramatic rise in pancreaticoduodenal venous glucagon concentration, which reached a peak 3 minutes after injection, and hyperglycemia was noted to occur soon thereafter. Endoportal infusion of secretin and pancreozymin for 20 minutes caused responses that were sustained but qualitatively identical to the responses noted after rapid injection of the hormones. The beta-cytotropic effect of secretin was abolished by the infusion of epinephrine. These results could not be attributed to the small degree of contamination of the enteric hormone preparations with insulin or glucagon, and it would appear that secretin, pancreozymin, and probably gastrin have insulin-releasing activity and that pancreozymin has, in addition, glucagon-releasing activity.The demonstration that these three hormones possess insulin-releasing activity suggests that there is in the gastrointestinal tract a chain of betacytotropic hormones from antrum to ileum that is capable of augmenting insulin secretion as required for disposal of substrate loads. It is suggested that the existence of this "entero-insular axis" prevents high substrate concentrations that would otherwise follow ingestion of large meals were the insular response entirely a function of arterial substrate concentration.