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The role of oestradiol in the regulation of LH release in the hen was studied by use of the anti-oestrogen, tamoxifen (ICI 46,474).

Intramuscular injection of laying hens with 2 or 4 mg tamoxifen on 2 successive days delayed or prevented the occurrence of the preovulatory release of LH and ovulation expected on day 3. Ovulation could be restored by i.v. injection of 20 pg LH releasing hormone (LH-RH). Tamoxifen at a dose of 1 mg affected neither the timing of the preovulatory release of LH nor ovulation. Treatment with 2 or 4 mg tamoxifen on 2 successive days reduced the effectiveness of an i.m. injection of progesterone to stimulate a release of LH. Injection of 1, 2 or 4 mg tamoxifen on 2 successive days significantly raised basal levels of LH in the blood at 24 h after the last injection. This was associated with an increase in the capacity of the pituitary gland to respond to an injection of synthetic LH-RH by a release of LH.

These studies suggest that oestradiol has at least two roles in the regulation of LH release in the hen. First, it maintains a low basal level of LH in the blood by reducing the responsiveness of the pituitary gland to LH-RH. Secondly, oestradiol has a facilitative role in the mechanism by which progesterone stimulates the preovulatory release of LH.