The purpose of this study was to determine the effects of morphine and naloxone on pulsatile release of LH in ovariectomized rats treated (or untreated) with ovarian steroids. Ovariectomized rats were given a subcutaneous injection of estradiol benzoate (20 µg) or estradiol benzoate (20 µg) and progesterone (10 mg) 3 days prior to experimentation. The rats were then given intravenous injections of naloxone (2 mg/kg), morphine (5 mg/kg), or 0.87% NaCl every hour for 3 h. For LH assays, 0.3 ml blood was collected via an atrial cannula 15 min after drug treatment and every 15 min thereafter for 3 h. Pulsatile LH release was suppressed by estradiol benzoate or the combination of estradiol benzoate and progesterone. Naloxone was able to counteract inhibition of pulsatile LH release by these steroids. These results suggest that the endogenous opioid peptides are involved in the negative feedback exerted by estrogen and progesterone on pulsatile LH release. Morphine had no effect on steroid inhibition of pulsatile LH release.