Regulation of the exocrine pancreatic secretion elicited by a meal in man is incompletely understood. In this study, we attempted to localize in the gastrointestinal tract areas that control postprandial trypsin secretion and to determine the effects of individual components of jejunal chyme on the meal-stimulated trypsin secretion. Trypsin outputs elicited by ingesting a mixed-nutrient meal and diverting it at the ligament of Treitz proximal to an occlusive balloon were compared with those occurring when the same meal was ingested, diverted at the ligament of Treitz, and immediately reinfused distal to the balloon, a procedure that exposed the entire gastrointestinal tract to chyme. Two different meals, one of semielemental and one of complex nutrients, were used with similar results. Trypsin outputs were similar whether or not jejunal chyme was diverted. In addition, no component--exogenous nutrients or endogenous secretions--of chyme reaching the jejunum after a meal further modified the trypsin secretion elicited by the gastroduodenal segment. This finding suggests that the gastroduodenal segment is sufficient to elicit the entire postprandial trypsin output and is the physiologic determinant of meal-stimulated trypsin secretion.