The effect of dietary soya bean trypsin inhibitor (SBTI) on rat pancreas, stomach, duodenum and two gastrointestinal hormones (gastrin and cholecystokinin (CCK] levels was investigated over a 21 day period. Rats were fed either a complete diet containing 17% protein as casein (control animals), or the same diet supplemented with 1.1% SBTI (test animals). After 21 days the weight of the pancreas in the test group was 90% higher than that in the controls. The stomach antrum, and duodenum showed no change. The gastrin content in the antrum in fasted rats increased after SBTI feeding whereas plasma gastrin levels remained the same. Dietary SBTI had no effect on fasting CCK levels either in duodenal mucosa or in plasma. The results obtained on gastrin values indicate that SBTI stimulates gastrin biosynthesis in the rat antrum but does not alter its release into the circulation. Results obtained on CCK values suggested the following hypothesis: the endocrine tissue of the duodenum, where CCK is synthesized and stored, represents a relatively large reservoir of this hormone. In addition the secretory capacity of this tissue is probably much higher then is required during normal physiological conditions. Therefore, this tissue does not undergo any adaptive changes after prolonged overstimulation by SBTI. The plasma CCK peak following SBTI intake is probably much higher than after control diet, but clearance of CCK from plasma is relatively fast. Consequently, the high levels of CCK after SBTI intake produce overstimulation of pancreatic secretion and in the long term alter the pancreatic function and morphology but return to normal levels in fasting state.