Relationship between neonatal treatment with estrogen and induction of mammary tumors by 7, 12-dimethybenz [a] anthracene (DMBA) in female rats was examined. Five-day-old female rats received intrahypothalamic implantation of micronellets of 0.4μg of estradiol-paraffin mixture or paraffin vehicle only (sham-operated control), or no operation (intact control). Induction of mammary tumor in these rats was examined after DMBA was given at 45 days of age intragastrically. The rats bearing the micropellets of the estradiol-paraffin mixture in the anterior to middle part of the hypothalamus became persistent vaginal estrus while the rats bearing the same pellets in the middle to posterior part of the hypothalamus showed regular cyclic changes in daily vaginal smears. Compared to the intact or sham-operated controls, mammary tumor appearance was delayed in the rats which received 0.4μg of estradiol intrahypothalamically and became persistent vaginal estrus (10 weeks vs. 15 weeks after DMBA administration, respectively). Since (1) the vagina of the rats with intrahypothalamic micropellets of estradiol-paraffin mixture opened at the same age as the intact or sham-operated controls and (2) the mammary rating of the former rats was the same as the latter, it can be concluded that the hypothalamus of the former rats has been changed by the steroid while the neonatal mammary glands have received little effect. Thus, the delay or suppression of mammary tumorigenesis in DMBA-treated rats by neonatal steroid administration is primarily attributable to the irreversible changes in the hypothalamo-pituitary-gonadal axis.