Examinations were made on the carcinogenic effects of a chemical compound, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and a physical carcinogen, whole-body irradiation with fission neutrons, on the gastrointestinal tract of male albino Sprague-Dawley rats. The carcinogens were used singly and together in order to investigate their possible synergistic effects on the induction of adenocarcinomas of the stomach and small intestine. Of the 13 animals treated with the chemical, MNNG, and living more than 9 months, 9 showed gross tumors (5 gastric and 4 duodenal), confirming the high incidence of gastrointestinal carcinomas induced by MNNG in the rat. There were no gastrointestinal tumors found after neutron exposure. When the 2 carcinogens were combined, no additivity or synergism occurred. After neutron irradiation, a dental syndrome with loss of incisor teeth was observed. The effect of neutron irradiation on subgingival pathology of the teeth is being investigated.