Raw soya flour (RSF) is thought to cause pancreatic hypertrophy in rats because trypsin inhibitor in the flour binds trypsin in the gut lumen and this lowering of free trypsin concentrations leads to the release of cholecystokinin (CCK). Intestinal enzyme concentrations were therefore studied in rats fed RSF for from one hour to 400 days to determine whether free trypsin concentrations were depressed during the period of pancreatic growth (up to six to eight weeks after starting RSF). Intestinal levels of enzymes were raised from one hour after starting the diet up to 400 days, indicating that stimulation of pancreatic secretion continued for as long as this diet was fed. Free trypsin concentrations were, however depressed only for the first 12 hours. It seems unlikely, therefore, that lowered free trypsin concentrations alone stimulate the release of CCK in these animals.