To evaluate whether the median eminence (ME) is a site of action of opioids on gonadotropin secretion, and to determinate the role of estradiol (E) and progesterone (P) in modifying these secretion in response to naloxone, we injected naloxone (1, 2.5 and 5 micrograms) into the ME in ovariectomized rats treated with estradiol benzoate (OVX+E) or ovariectomized rats treated with estradiol benzoate plus progesterone (OVX+E+P), and measured LH and FSH serum levels 10 and 30 min postinjection. Naloxone injection at doses of 2.5 and 5 micrograms significantly increased serum LH levels in OVX+E animals 10 and 30 min postinjection, whereas naloxone injection at all doses in OVX+E+P rats did not have any significant effect on LH secretion 10 and 30 min postinjection, when compared with values obtained in controls injected with saline. The injection of naloxone into the ME did not modify FSH secretion. We conclude that naloxone, acting within the ME, can elicit LH release in OVX+E rats, while it abolishes this effect in OVX+E+P rats.