Effects of neonatal androgenization on 7,12-dimethylbenz[a]anthracene (DMBA)-induced endometrial carcinogenesis in Sprague-Dawley rats are studied. Testosterone propionate (1.25 mg) was injected subcutaneously into 110 2-day-old rats. At 8 weeks of age, a pellet containing 0.2 or 1mg DMBA was introduced into each uterine horn of both neonatally intact control rat (NR) and neonatally androgenized rat (ASR). All rats were killed at 24 or 48 weeks of age. The results were as follows. Vehicle group: No malignant uterine tumor was observed during both 16- and 40-week observation periods in either 122 horns of NR or 98 of ASR. 0.2 mg DMBA treated group: No malignant uterine tumor was detected in either 80 NR or 72 ASR during a 16-week observation period. Although three malignant uterine tumors were found among 72 NR during a 40-week observation period, no malignant uterine tumor was observed in 30 ASR. 1mg DMBA treated group: Although one malignant uterine tumor of 16 NR was found during a 16-week observation period, no malignant uterine tumor was observed in 20 ASR. Four malignant uterine tumors of 168 NR induced by DMBA were histologically 2 squamous cell carcinomas and two sarcomas. As mentioned above, although four malignant uterine tumors were induced by DMBA in 168 NR, no malignant uterine tumor was found in 122 ASR. These results seem to indicate that neonatal androgenization suppressed the induction of malignant rat uterine tumors by DMBA.