We used the combined steroid autoradiography-immunocytochemical method to determine whether estradiol- or dexamethasone-concentrating cells contain endogenous opioid peptides. Ovariectomized-adrenalectomized female rats were given highly radioactive doses of 3H-estradiol or 3H-dexamethasone, then sacrificed to demonstrate nuclear steroid binding. Autoradiograms were prepared, exposed for 2–12 months, photodeveloped, and fixed; immunocytochemistry was carried out on the same sections using antibodies to β-endorphin or dynorphin A (1–17). In the medial basal hypothalamus, many estradiol- and some dexamethasone-concentrating neurons were found intermingled with β-endor-phin or dynorphin-immunoreactive neurons. Of the β-endorphin-immunoreactive neurons in the medial basal hypothalamus, 4% concentrated estradiol in their nuclei. In addition, a subset of β-endorphin-immunoreactive cells in the anterior pituitary concentrated estradiol in their nuclei. Although none of the β-endorphin-immunoreactive neurons in the medial basal hypothalamus concentrated dexamethasone in their nuclei, many of the β-endorphin-immunoreactive cells in the anterior pituitary did. Of the dynorphin-immunoreactive neurons in the medial basal hypothalamus, 10% concentrated estradiol in their nuclei. These data are consistent with the hypothesis of a genomic effect of estradiol on a particular subset of medial basal hypothalamic neurons that produce endogenous opioid peptides.