Antrocolic transposition in four dogs with Heidenhain pouch and gastrojejunostomyresulted in a marked increase in fasting plasma gastrin concentration and sustained highacid secretion, closely related to gastrin levels. A marked production of pepsin output could not be correlated with plasma gastrin. Reduction in plasma gastrin concentration was more than twofold less pronounced than the reduction in acid output for 1 and 2 U/kg-h, while the 0.5 U/kg-h no effect was noted. For both acid output and gastrin concentrations, close correlations were noted between presecretion level and remaining level upon secretin infusion. Despite the reduction in the secretory volume of the pouch, 0.5 and 1 U/kg-h of secretin induced a 1.5- and 2-fold increase in pepsin output, respectively. Two untis per kilagram-hour decreased the secretory volume as well as the pepsin output. If the physiological release of secretin in dogs does not exceed the equivalent of the lowest dose studied, our results would indicate that acid inhibition is a physiological activity of secretin, while the effect on circulating gastrin concentration seems to be phamacological.