The effects of oral administration of ethylenethiourea (ETU) and sodium nitrite on endometrial adenocarcinoma development was investigated in female Donryu rats. At 10 week of age, groups 1 and 3 received a single dose of polyethylene glycol (PEG) into the uterine cavity, while groups 2 and 4 were given N-ethyl-N-nitrosourea (ENU) (15 mg/kg) solution, dissolved in PEG, in the same manner. ETU (80 mg/kg) and sodium nitrite (56 mg/kg) dissolved in distilled water were orally given to animals in groups 3 and 4 once a week from 11 to 51 weeks of age. Groups 1 and 2 received the vehicle alone. At termination (52 weeks of age), endometrial adenocarcinomas were observed in 29, 13 and 57% of rats in groups 2, 3 and 4, respectively, but not in group 1. Persistent estrus appeared earlier in groups 3 and 4 than in group 1. In groups 3 and 4, hyperplastic and neoplastic lesions of the digestive tract, especially the forestomach, were also observed. The results indicate that N-nitroso ETU formed in the stomach by concurrent oral administration of ETU and sodium nitrite, itself induces endometrial adenocarcinomas by its mutagenic action, as well as promoting their development after ENU-initiation, presumably by influencing the hormonal balance.