Cholecystokinin (CCK) has trophic actions on abdominal viscera. To determine whether CCK enhances malignant growth in a similar fashion, we gave hamsters CCK together with di-isopropanol nitrosamine (DIPN), a known pancreatic and hepatic carcinogen. After 40 weeks of injections, those animals receiving both DIPN and CCK developed no cancers, while the control animals receiving DIPN alone developed pancreatic and hepatic carcinomas. This suggests that CCK inhibits carcinogenesis in this model. Although the mechanism of this effect is unknown, some implications for human carcinogenesis are testable by currently available methods.