Four experiments were conducted to investigate the effect of naloxone, an opioid receptor antagonist, on pituitary LH secretion in Holstein cows during two periods after parturition and two phases of the estrous cycle. In experiment 1, 24 cows (12 primiparous; 12 multiparous) received either saline (n = 12) or 1 mg/kg naloxone (n = 12) i. v. at 14 1 days postpartum. Blood samples were collected at 15-minute intervals for 2 hours before and 2.5 hours after naloxone or saline. Serum LH concentrations increased (P < .05) in response to naloxone injection in both primi- and multiparous cows. Saline injection did not affect LH concentrations. In experiment 2, 27 cows (13 primiparous; 14 multiparous) received either saline (n=14) or 1 mg/kg naloxone (n=13) i. v. at 28 ± 1 days postpartum. Blood samples were collected as in the previous experiment. Naloxone did not affect serum LH concentrations in either primi- or multi-parous cows at 28 days postpartum. In experiment 3, estrous cycles were synchronized via prostaglandin administration (25 mg) in 22 cows (10 primiparous; 12 multiparous). Cows received either saline (n=11) or 1 mg/kg naloxone (n=11) Lv. during the luteal phase of the estrous cycle. Blood samples were collected as in the previous experiments. Luteinizing hormone concentrations were not affected by naloxone in either primi- or multi-parous cows during the luteal phase of the estrous cycle. In experiment 4, the same cows used in experiment 3 received a second dose of prostaglandin (25 mg). Thirty-six hours later, during the follicular phase of the estrous cycle, the cows received either saline (n =9) or 1 mg/kg naloxone (n = 11) i. v. Naloxone increased (P < .05) serum LH concentrations in both primi- and multi-parous cows in the follicular phase. These results suggest that LH release in the early postpartum dairy cow is regulated, at least in part, by endogenous opioid pep tides , and the ability of naloxone to affect LH secretion may change as days postpartum increases, perhaps due to changes in degree of inhibition by endogenous opioid peptides, and (or) changes in serum progesterone concentration due to onset of ovarian activity during postpartum period. It appears that the modulation of LH secretion may be mediated via opioids during the follicular phase of the estrous cycle. However, an opioid-mediated mechanism for modulation of LH secretion was absent or overridden by progesterone feedback during the luteal phase of the estrous cycle.