Secretin and somatostatin are two peptides released by H+ ions. The fact that blood levels of these peptides increase during the postprandial period makes them the most probable candidates for gastric acid secretion retrocontrol . The aim of our study was to determine whether a potentiation of inhibitory effects exists when the two peptides are administered simultaneously. Seven dogs were provided with a gastric fistula. Gastric acid secretion was stimulated by intragastric infusion of a bactopeptone solution or by pentagastrin (1 microgram X kg-1 X h-1). Acid outputs and gastric emptying rates were measured at regular intervals. The tests were repeated during intravenous infusion of secretin (0.125 micrograms X kg-1 X h-1), somatostatin (0.2 micrograms X kg-1 X h-1) or both peptides together at the same doses. Both secretin and somatostatin inhibited acid secretion and gastric emptying. The inhibition was not greater when both peptides were administered together. Under the conditions used, no potentiation or additive effects between the two peptides were observed on gastric acid secretion or gastric emptying.