Summary. The opioid antagonist WIN-44441-3 (WIN-3, Sterling-Winthrop) caused significant increases in LH secretion in ovariectomized ewes treated with progesterone but not in ovariectomized animals treated with oestradiol-17β. In the non-breeding season, plasma LH concentrations in ovariectomized ewes without steroid therapy, given oestradiol-17β or oestradiol-17β and progesterone together were not affected by treatment with WIN-3 on Day 6 after ovariectomy (there was a significant increase in LH as a result of WIN-3 treatment 13 days after ovariectomy in sheep given no steroid therapy). However, WIN-3 treatment of ovariectomized sheep given progesterone resulted in a significant increase in plasma LH. WIN-3 was ineffective when given to intact ewes treated with progesterone during the non-breeding season. With ovariectomized sheep during the breeding season there was again no response to WIN-3 at 6 days after ovariectomy in sheep given oestradiol-17β, but significant LH elevations in animals given no steroid, those given progesterone and those given progesterone + oestradiol-17β. The lack of an LH response to WIN-3 in ovariectomized sheep treated with oestradiol-17β did not result from a reduced pituitary response to GnRH since such animals responded normally to exogenous GnRH treatment. Overall, these results are consistent with the idea that, irrespective of the time of year, progesterone exerts negative feedback upon LH release at least in part through an opioidergic mechanism, whereas oestradiol-17β exerts negative feedback through steps unlikely to involve opioids. Progesterone can override the effect of oestradiol-17β during the breeding season only. Further, there appears to be a steroid-independent opioid involvement in LH suppression, operating at both times of year.