To investigate the role of endogenous opioid peptides (EOP) in the inhibitory control of LH secretion in the ram, the acute effects of naloxone (opioid antagonist) on episodic LH secretion were measured in rams at different stages of a reproductive cycle induced by treatment with melatonin. Groups of SCGx rams (functionally pinealectomized) and pineal intact rams were housed under long days (16 h light: 8 h darkness) and treated with alternating 16 week periods with exogenous melatonin (continuous melatonin from silastic implant) and 16 week periods with no exogenous melatonin for 3 or 4 consecutive cycles. The LH response to naloxone (1.6 mg/kg i.v.) was measured at 2-4 week intervals on 9 occasions during one of the treatment cycles. The periodic treatment with melatonin resulted in a clearly defined cycle in the plasma concentrations of LH, FSH, testosterone and prolactin, and associated changes in size of the testes, intensity of the sexual skin flush and moulting of the pelage; maximum size of the testes occurred 8-16 weeks after the start of each melatonin treatment. Naloxone induced an increase in plasma LH concentrations at all times but the response varied in relation to the stage of the melatonin-induced reproductive cycle. During testicular recrudescence, naloxone induced large increases in mean LH concentration (low frequency, high amplitude LH pulses), at the peak of the reproductive cycle naloxone induced smaller increases in plasma LH (high frequency, low amplitude pulses) and during testicular regression naloxone induced only minor increments in plasma LH. The results are consistent with the role of EOP in the inhibitory control of LH secretion with this system most active during the sexually active phase of the reproductive cycle.