Pancreatic tumorigenesis of azaserine by a single i.p. injection after partial pancreatectomy in male Wistar rats was studied. Pancreatic lesions developed in all rats 52 wk after the administration of azaserine at doses of 50 mg (group 1) or 100 mg/kg body wt (group 2). Histologically, there were hyperplastic nodules in 4 of 12 rats and 4 of 7 rats in groups 1 and 2, respectively, adenomas in 100% in both groups, and adenocarcinomas in 2 of 7 rats of group 2. It appears that pancreatic regeneration induced by partial pancreatectomy enhances the azaserine tumorigenesis.