Indomethacin, a known inhibitor of prostaglandin (PG) synthesis, blocked the estradiol-17β (E2-17β)-induced release of luteinizing hormone (LH) in sheep. This finding provides evidence of an obligatory role for endogenous PG synthesis in the mechanism of LH release. In addition it suggests a possible use for PG inhibitors in fertility control through the blockade of LH release and hence ovulation.