These experiments were performed to analyze whether treatments of ovariectomized female rats with ovarian steroid regimens able to induce either an increase (positive feedback effect) or a decrease (negative feedback effect) of serum levels of luteinizing hormone (LH) have some impact on the characteristics of mu-opioid binding sites in circumscribed areas of the brain. The increase of serum levels of LH elicited by a treatment with estradiol benzoate (EB) plus progesterone (P; positive feedback effect) was accompanied by a significant decrease in the number of mu-binding sites in the hypothalamus and in the corpus striatum. The decrease in serum levels of LH induced by a treatment with EB alone (negative feedback effect) brought about a significant increase of the number of mu-binding sites in the thalamus and in the hippocampus. These results seem to suggest that the release of LH induced by EB plus P may involve a decrease of hypothalamic mu-binding sites. Apparently, the inhibitory effect on LH release exerted by EB alone does not involve any change of the density of these binding sites in the hypothalamus.