Various doses of diethylstilbestrol (DES) were administered to rats once at birth. Thereafter, at 50 days after birth, the rats in all groups were given 10 mg 7, 12-dimethylbenz[a]anthracene (DMBA) and undergone necropsy at 300 days after birth. The incidence of mammary carcinomas (MCs) were 50, 54, 91, 39, 19% at 175 days after birth, and 77, 87, 100, 85, 75% at necropsy in the 0, 0.1, 1, 10, 100 microg groups, respectively. The incidence of rats without corpus luteum were 0, 0, 0, 30, 100% at 50 days after birth, and 0, 40, 53, 93, 100% at necropsy in the 0, 0.1, 1, 10, 100 microg groups, respectively. Observation of the whole mount specimens showed a higher number of terminal end buds (TEBs) in the 1 microg group and a lower number in the 100 microg group compared with the control at 50 days after birth. It suggested that the administration of a relatively low dose (1 microg) of DES during neonatal period may increase TEBs, thus resulting in a stimulatory effect on the initiation of MCs.