Sex steroids may modulate the secretion of beta-endorphin (beta-EP). Naloxone (Nal), an opioid antagonist, has been used as a probe of central opioid activity. Nal-evoked responses of PRL and LH were evaluated in the midluteal (ML) and late follicular (LF) phases of ovulatory women (Pre) and compared to responses of oophorectomized women before and after the administration of conjugated estrogens (CE) and again after CE and progestin administration. In the ML and LF phases, serum LH increased significantly (P less than 0.05 and P less than 0.01, respectively) during Nal infusion for 4 h, while PRL did not change. In oophorectomized women, there were no significant changes in LH or PRL during Nal infusion. After 3 weeks of CE treatment (1.25 mg daily), LH increased during Nal infusion (P less than 0.05), as did PRL (P less than 0.01). After treatment with CE and medroxyprogesterone acetate (MPA), LH and PRL both increased (P less than 0.05 and P less than 0.01, respectively). The area under the LH curve during Nal infusion after CE and MPA treatment was greater than that after CE alone. Both of these responses were comparable to those of the LF and ML phases of Pre women. During Nal infusion, LH pulse frequency increased in the ML compared to the LF phase of the cycle and, in oophorectomized women, was greater after CE and CE with MPA treatment compared to pretreatment values (P less than 0.05). LH amplitudes during Nal infusion were highest in the ML phase and after CE and MPA treatment in oophorectomized women, and these LH amplitudes were similar. No correlation was found between peripheral plasma beta-EP and Nal-evoked LH responses. No differences were evident in plasma beta-EP levels between Pre and oophorectomized women. In conclusion, 1) endogenous opioid activity is low in oophorectomized women; 2) treatment with estrogen increases opioid activity, and the addition of a progestin increases this activity further; and 3) these data support the contention that sex steroids exert a profound influence on endogenous opioid activity.