The author investigated the hyperplastic and hypertrophic effect of caerulein in the pancreas of normal syrian golden hamsters, and the promoting effect of experimental pancreatic cancer in hamster induced by N-nitroso-bis (2-hydroxypropyl) amine (BHP). The results are as follow: Repeated subcutaneous injections of caerulein in every 12 hours for 10 days elicited a marked trophic effect on the pancreas, characterized by increased pancreatic weight and pancreatic weight/DNA ratios with an enhanced content of DNA and amylase in the pancreas in treated hamsters. DNA synthesis, as measured by histoautoradiography of tritiated thymidine labeled tissues, was increased in pancreatic acinar but little in islet nor in ductal cells. Weekly subcutaneous administration of BHP with caerulein brought pancreatic carcinomas earlier and in higher incidence than BHP alone. The majority of induced carcinomas were well differentiated adenocarcinomas, and acinar cell carcinoma was seen in neither groups. In addition, a further investigation was performed in search target cells of both BHP and caerulein. Repeated injections of caerulein in every 12 hours for 5 days before one shot of subcutaneous BHP administration led to increase both mitotic and labeling index using tritiated thymidine in most acinar cells. These results suggested that caerulein has a trophic action on the pancreas, and acts as a promotor in experimental pancreatic carcinoma.