To elucidate a possible secondary gastric hypersecretion due to the lack of secretin response after biliary diversion from the duodenum, gastric acid outputs and serum secretin concentrations, basal and in response to feeding, were measured before and after diversion of bile from the duodenum to the jejunum in dogs prepared with Heidenhain pouches. Both 24-hour and postprandial Heidenhain pouch acid outputs increased significantly after biliary diversion compared with the preoperative controls(p<0.01). Increases in serum secretin concentrations caculated as integrated responses after feeding were also greater after biliary diversion compared with the preoperative controls(p<0.01). It is suggested that the changes in serum secretin levels after biliary diversion seem to be the result, rather than the cause, of gastric hypersecretion and if the mechanism of gastric hypersecretion after biliary diversion is considered in view of the loss of gastric inhibitory hormones, some hormones other than secretin should be suspected and studied.