The effects of dietary soybean trypsin inhibitor (SBTI, Kunitz type) or repeated i.p. injections of 95% pure cholecystokinin (CCK-39) on rat pancreas were investigated in a 10-day experiment. SBTI and CKK -39 induced similar increases in pancreatic weight, which led to both cellular hypertrophy and hyperplasia. Trypsin and chymotrypsin activity increased with an increase in pancreatic weight. Amylase activity increased only after CCK-39 injection, whereas lipase activity was not affected by either SBTI or CCK-39 treatment. After both treatments, insulin content showed only a slight tendency to increase, whereas glucagon content was not different from controls. The results indicate that SBTI and CCK-39 mainly exert their effects on the exocrine pancreas in a similar but not identical manner. It is therefore suggested that SBTI is not only a potent stimulator of the secretion of CCK activity but also of other unidentified gastrointestinal factor(s).