GIH secretin bolus (2 CU/kg) and infusion (3 CU/kg/h) have been randomly compared in 9 ZES patients and 10 age-matched DU patients. Serum gastrin and gastric acid variations were studied before and after either mode of secretin administration in the same individuals. Plasma secretin modifications were monitored in parallel. In both ZES and DU, secretin bolus and infusion induced similar gastrin responses (maximal changes and integrated responses). However, secretin infusion had a greater effect on acid output than bolus: larger inhibition in DU and larger increase in ZES. The additive diagnostic value of gastric acid secretion study during a secretin provocation test, as already reported, favors the use of 3 CU/kg/h secretin infusion over that of 2 CU/kg secretin bolus.