The influence of methionine-enkephalin analogue (FK 33-824) administration upon prolactin and luteinizing hormone (LH) secretion in follicular phase gilts was studied. In experiment I, gilts, selected at random, received intravenous bolus injections of FK 33-824 or saline at 4 μg kg −1 body weight (BW) on Days 16 or 19 of the estrous cycle. Peripheral blood samples were collected every 15 min from 3 h before to 6 h after administration of FK 33-824 or saline. In experiment II, gilts were infused for 4 h with saline ( N = 4) or FK 33-824 at a rate of 6.2 μg kg −1 BW h −1 ( N = 4) on Day 19 of the estrous cycle. At the onset of continuous infusion, treated gilts received a bolus intravenous injection of FK 33-824 at a dose of 4 μg kg −1 BW. On Day 19 of the cycle, four additional gilts were also infused for 4 h with FK 33-824 as in experiment II. Two of the gilts received four injections of naloxone (2 mg kg −1 BW) at 1-h intervals beginning 15 min before the infusion of FK 33-824 and the remaining two gilts were injected with 100 μg of gonadotropin-releasing hormone (GnRH) 2 h after the beginning of FK 33-824 infusion. Blood samples were collected every 15 min for 4 h before, during and 4 h after infusion. Bolus injections of FK 33-824 resulted in increased ( P < 0.05) prolactin concentrations on Day 19 of the estrous cycle but not on Day 16. Bolus injections of the opioid agonist did not influence LH profiles. Continuous infusion of FK 33-824 on Day 19 of the cycle elevated ( P < 0.05) plasma prolactin concentrations and decreased ( P < 0.05) LH concentrations and number of LH pulses. These effects of FK 33-824 were reversible by naloxone. Release of LH in response to GnRH injection during continuous infusion of FK 33-824 was observed. The results suggest an involvement of the opioid system in regulation of prolactin secretion and revealed an alteration of LH secretion when gilts were subjected to prolonged opioid influences during the late follicular phase.