The regional distribution of estrogen receptors (ER) was studied in the brain of the female rat over the peripubertal period. Increases were observed in nuclear ER occupation as well as cytosolic progestin receptor induction in the period leading up to the time of vaginal opening, consistent with an increased availability of estrogen to the brain. ER binding capacity, however, was remarkably similar in pre- and postpubertal rats. Using a microdissection-based nuclear ER assay, small increases in the total ER content of the hypothalamic ventromedial and arcuate nuclei were observed in the female at around the time of the onset of reproductive cyclicity. In vivo autoradiographic studies confirmed the existence of a small increase in estrogen binding in these two brain regions at around the time of vaginal opening, as well as transient increases in estrogen retention within the preoptic area and bed nucleus of the stria terminalis. These changes in estrogen binding in the brain may play a role in pubertal alterations in the feedback sensitivity of the brain to circulating estrogen.