Gastric and duodenal adenocarcinomas were induced by N-methyl-N′-nitro-N-nitrosoguanidine (NG) in 3 strains of inbred and 1 strain of outbred rats. The carcinogenic effect of NG was limited to the upper digestive tract, but a high incidence of nonmalignant biliary cysts was also observed. The successful induction of gastroduodenal neoplasms by NG in inbred animals was at variance with previous reports. Such induction is essential, however, for tumor transplantation or immunologic studies. The choice of inbred strain may be important, since carcinoma incidence varied significantly among strains.