Estrogen-Induced Release of LH in the Ovariectomized Ewe: Independence of Time of Day1

The time interval between injection of estrogen and the resulting LH surge was determined in ovariectomized ewes. All experiments were conducted from October through February. Ovariectomized ewes which had previously received either progesterone (P) or no other treatment were injected with 50 µg of estradiol benzoate (EB) to induce LH release. LH levels in the serum were measured by radioimmunoassay. In the first experiment two groups of P-treated ewes were injected with EB. Eight h later one group was given another injection of EB and the other group was given oil. There was no significant difference in the intervals between the first dose of EB and peak levels of LH (16.3 ± 1.1 vs 18.0 ± 1.7 h) in the two groups. When P-treated ewes were given EB at either 0600 or 1800 h peak levels of LH in the serum occurred 17.4 ± 1.2 h and 16.8 ± 0.7 h later, respectively. In a third experiment ovariectomized ewes were given EB at 0900 h, at 1200 h, or at both times. All showed peak levels of LH approximately 14 h after the initial injection of EB. These results demonstrate that E-induced LH release occurs in the ovariectomized ewe, in contrast to the rat, at a relatively constant interval after injection of EB rather than at a specific time of the day.