Two experiments were undertaken to induce uterine cancer in rats by intrauterine administration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in a total of 120 inbred female ACI rats, 5 weeks old. In Experiment I, 55 rats were divided into 3 groups and given a single injection of MNNG 50 mg (Group 1) or 100 mg (Group 2)/kg body weight emulsified in olive oil or olive oil alone (Group 3) into the uterine canals after the laparotomy. Uterine corpus tumors including endometrial adenocarcinomas were developed in 35.3% (Group 1) and 16.7% (Group 2) of the effective animals. Endometrial adenomatous hyperplasia which was considered to be a precancerous lesion, was observed in a few rats of Group 2. No tumors nor hyperplastic lesions were seen in the control group. (The incidence of tumors in the uterine corpus was significantly higher in the animals of Group 1 or 2 than in those of Group 3 (P less than 0.05]. In Experiment II, 65 rats were divided into 4 groups and treated as follows; Group 1: Rats were given intravaginal detention of absorbent cottons dipped with 0.5 mg of MNNG dissolved in 0.2 ml of olive oil for a week. The treatment was repeated 20 times once every two weeks. Group 2: Animals were treated by intravaginal detention of cottons containing 10% acetic acid or MNNG in alternate weeks 5 times each, and then the detention of MNNG-cottons 15 times every other week. Group 3: Rats were given the absorbent cottons containing olive oil alone 20 times biweekly.(ABSTRACT TRUNCATED AT 250 WORDS)