1. 1. The secretory responses of hepatic bile and exocrine pancreas by intraduodenal infusion of propionate (PA), 3C1-PA, 2C1-PA, 3Br-PA and 2Br-PA solutions were examined in anesthetized piglets. 2. 2. Pancreatic juice and protein secretions were enhanced by infusion of PA and PA analogue solutions of pH 2.0 following increase of plasma secretin level but not pH 7.0. The order of response time was as follows: 3Br-PA > 2Cl-PA > 2Br-PA > 3Cl-PA > PA. 3. 3. The response of bile flow depended on endogenous secretin and showed almost the same pattern as that of pancreatic juice secretion. 4. 4. The results suggested that pancreatic exocrine secretion via endogenous secretin was not always dependent on the dissociation constant of weak acids.