Sexual dimorphisms in the hypothalamus are mediated in several cases by local aromatization of androgens to estrogens during the perinatal period. In this series of experiments the contributions of the two estrogen receptors, (ER) and (ER) to the differentiation of the sexually dimorphic subpopulation of dopaminergic neurons in the anteroventral periventricular area (AVPV) was examined. In the first experiment, numbers of tyrosine hydroxylase (TH) immunoreactive (-ir) AVPV neurons in ER knockout (ERKO) and wild-type (WT) mice of both sexes were measured. In the second experiment, the average number of TH-ir neurons in the medial portion of the AVPV in ER knockout (ERKO), ERKO, double ER knockout (ERKO) and WT mice of both sexes was calculated. In both experiments TH-ir cell numbers were sexually dimorphic as expected, with female individuals of all genotypes exhibiting more TH-ir neurons than WT males. Interestingly the average number of TH-ir neurons in all knockout males was significantly higher than in WT male littermates. In fact, TH-ir cell numbers in all knockout males were equivalent to females. In a final experiment, C57BL/6J female mice were treated during the first three postnatal days with either estradiol, or a specific agonist for one of the two ERs. Additional male and female pups received vehicle injections. Treatments with estradiol or either ER-specific agonist significantly reduced the number of TH-ir AVPV neurons in female brains. Our data demonstrate that both ER and ER are involved in the sexual differentiation of the AVPV in mice.