This study was designed to test the hypothesis that melatonin would intensify daily LH release after central blockade of the opiate receptors in sexually active ewes. The intracerebroventricular infusions of vehicle (control), melatonin, naloxone and melatonin in combination with naloxone were made in ewes in the luteal phase of the estrous cycle, from 2:00 P.M. to 5:00 P.M. Blood samples were collected from 11:00 A.M. to 8:00 P.M. at 10-min intervals. The mean plasma LH concentrations were measured before, during and after the infusions. The frequency and amplitude of LH pulses were determined during the whole experimental period. The LH concentrations recorded during melatonin or naloxone infusions were significantly higher than the concomitant concentration in vehicle-infused animals. The mean LH pulse amplitude in melatonin- and naloxone-treated ewes was also significantly higher than in controls. The LH concentration measured during the combined infusion of melatonin and naloxone was significantly higher than that during vehicle infusion. The LH concentration recorded in turn after the treatment was significantly higher than the concomitant concentrations in vehicle-, melatonin- and naloxone-infused animals. The mean LH pulse amplitude in this group was significantly higher than in the vehicle-infused group. These results indicate that blockade of the opiate receptors within the CNS facilitated effective stimulation of daily LH secretion by exogenous melatonin. In conclusion, a relationship between melatonin and endogenous opioid peptides may be crucial in enabling melatonin to exhibit stimulatory action on LH secretion during the luteal phase of the estrous cycle in ewes.