A pair of micropellets of a 1% or 10% estradiol (E2)-paraffin mixture (containing 0.2 or 2 mug E2, respectively) or paraffin alone were implanted subcutaneously or intracerebrally in 5-day-old female rats. Animals given 10% E2 pellets became sterilized regardless of loci of the pellets. Of those which had received 1% E2 pellets, only animals with micropellets in the anterior hypothalamus became sterilized. It is suggested that neuronal components which are affected irreversibly by neonatal estrogen treatment are localized in the anterior hypothalamus.