N-methyl-N'-nitro-N-nitrosoguanidine (NG) in drinking water (83 mg/l) results in a high incidence of gastric adenocarcinoma in random-bred Wistar rats. When NG was discontinued after 25 weeks, 61% of animals developed cancer and after 52 weeks the incidence was 73%. In a Wistar-Mai-Furth group (39th generation) invasive adenocarcinoma was found in 12% of rats ingesting NG for 25 weeks and in 10% exposed for 52 weeks. No adenocarcinomas and only two sarcomas were found in 50 Buffalo-Mai rats (56th generation) receiving NG for 52 weeks. Varying the dose and frequency of NG administration did not overcome host resistance. Host resistance to gastric adenocarcinogenesis induced by NG may represent a new advantage of this animal model.