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Single injections of 50 µg estradiol-17β (E₂β) into ovariectomized sheep caused biphasic changes in plasma luteinizing hormone (LH). An initial 8-h period of LH inhibition was followed by a period (12–20 h after E₂β) of facilitated LH release. Pituitary LH responsiveness to small dosages of synthetic gonadotropin-releasing hormone (GnRH) was tested repeatedly at 2-h intervals during the time periods when plasma LH was inhibited and when it was facilitated. Reduced sensitivity to GnRH (91% decrease) characterized only the initial 4 h of the inhibitory period, suggesting that E₂β suppressed endogenous LH-releasing factor (LRF) during the latter part of the inhibitory period. Hypersensitivity (2-fold increase) to GnRH was briefly observed at the beginning of the period of E₂β-facilitated LH release. This transient and modest hypersensitivity does not completely account for the very large E₂β-induced increases in plasma LH. Therefore, E₂β probably increased endogenous LRF during the period of facilitated LH release.

The ability of lactating rats to secrete surges of LH in response to exogenous estradiol-1 7Li (E2j3) was examined. Lactating rats with varying numbers of pups (0-16) were submitted to 1 of 3 treatments: 1) ovariectomy on the day of parturition; 2) ovariectomy plus E2#{216} administration; or3) E #{216}administration. Ovariectomized