Oral administration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) to hamsters at a concentration of 50-83 μg/ml in the drinking water resulted in a high incidence of tumours in the glandular stomach. Short-term administration of MNNG for 4-6 months resulted in more adenocarcinomata in the glandular stomach than long-term administration for 7-8 months. One case of metastasis of an adenocarcinoma of the glandular stomach to the liver and 2 cases of metastasis to the regional lymph nodes were found. Spindle cell sarcomata in the glandular stomach and adenocarcinomata in the duodenum were also often produced.Oral administration of MNNG at the very high concentration of 500-2000 μg/ml induced a hepatic cell carcinoma, intrahepatic bile duct carcinomata, bile duct cystadenomata and cystic dilatation, and a haemangioma in the liver but no tumour in the glandular stomach.Sequential morphological studies on the glandular stomach of hamsters receiving 50 μg/ml of MNNG in the drinking water showed 3 stages of change of the mucosa. The mucosa became atrophic and eroded in the first 16 weeks. Irregular atypical glands developed at the margins of erosions and proliferation of spindle cells in the submucosa were found after 18 weeks. Spindle cell sarcomata developed in animals after 20 weeks. Adenocarcinomata developed between 25 and 32 weeks.