We examined the effect of estrogen on the expression of aryl-hydrocarbon receptor (AhR) and two types of AhR nuclear translocator (Arnt1 and Arnt2) mRNAs in the hypothalamus of ovariectomized rats. Northern blotting demonstrated that, in the mediobasal hypothalamus, a subcutaneous injection of 20 mug estradiol benzoate (E-2) significantly increased the expression of Arnt2 mRNA, but induced no significant changes in the expression of AhR and Arnt1 mRNAs. The expression of Arnt2 mRNA was significantly increased at 4, 24, and 72 h after the injection. Immunocytochemical study revealed that the number of Arnt2 immunoreactive cells was also significantly increased at 72 h after the injection. Conversely, in the preoptic area, injection of E-2 did not cause significant changes in the expression of any of the three mRNAs. These observations suggest that estrogen regulates Arnt2 expression in the mediobasal hypothalamus and modulates the toxic action of dioxins in rats. (C) 2003 Elsevier Science (USA). All rights reserved.