The density of µ-opiate receptors located in the medial preoptic area (MPOA) of the rat hypothalamus is cyclical and sexually dimorphic. The hormonal regulation of MPOA µ-receptors was examined in ovariectomized rats treated with a variety of hormone regimens. In experiment 1, animals received acute estradiol (E2), progesterone (P), or prolactin (PRL), or E2 followed in 48 h by either P or vehicle by subcutaneous injection. Brains were removed 3 h after the final injection. In experiment 2, animals were implanted with empty or E2-filled Silastic capsules, and received either P or vehicle by injection 48 h later, at which time E2 capsules were removed. One group received E2 implants which remained in place following sham removal surgery. Brains and trunk blood for radioimmunoassay of E2 and P were collected 3, 27 or 51 h after the final injection. Frozen brain sections were prepared, incubated in [3H]D-Ala2, MePhe4, Gly-ol5-enkephalin, which selectively labels µ-receptors, and analyzed using quantitative receptor autoradiography. P treatment significantly increased MPOA µ-receptors, but only 27 h after E2 priming. Neither shorter P exposure, nor E2, P or PRL alone affected MPOA µ-recepter density. Following this delayed E2, P-induced increase, µ-receptor density subsequently decreased in the presence or absence of E2. The results suggest that E2, P treatment produces a gradual and transient increase of MPOA µ-receptor density. The subsequent decrease of receptor density is independent of the presence of E2 and may be related to receptor turnover. The time course of this effect is consistent with that of the estrus cycle. Such hormone-induced regulation of MPOA µ-receptor density could influence the physiologic effects of opiates on gonadotropin secretion and reproductive behavior in cycling