Re-establishment of stimulatory estradiol effects on luteinizing hormone secretion in long term ovariectomized rats

The ability of a single injection of estradiol benzoate (EB, 20 micrograms) to induce proestrus-like surges of luteinizing hormone (LH) on the day following EB administration was tested in long term ovariectomized rats which had been pretreated with EB (20 micrograms) and/or progesterone (P, 2.5 mg). LH afternoon surges could be evoked only in those animals which had previously received both ovarian hormones, EB and P. Pretreatment with a single hormone was ineffective. Moreover, in rats preinjected only with P the LH surge was shifted from afternoon to morning hours.