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Plasma oestradiol and delayed ovulation after administration of sodium pentobarbitone to pro-oestrous 5 day cyclic rats.

Injection of sodium pentobarbitone (Nembutal) into rats between 12.00 and 13.00 h on the day of pro-oestrus was fully effective in blocking the expected ovulation. In 75% of the rats, ovulation of the present generation of large follicles occurred 24 h later (delayed ovulation). Injection of Nembutal between 12.00 and 13.00 h on the day of pro-oestrus and at the same time on the subsequent day was fully effective in blocking the ovulation twice: in only two rats out of 13 did ovulation of the present generation of follicles still occur. When unilateral ovariectomy was performed immediately after injection of a single dose of Nembutal into pro-oestrous rats, delayed ovulation was significantly inhibited. However, after inhibition of ovulation by either two injections of Nembutal or one injection and unilateral ovariectomy, delayed ovulation could be induced by treatment with a small dose of oestradiol benzoate during the Nembutal-induced anaesthesia. It thus seemed that delayed ovulation failed because of disruption of oestrogen production after administration of Nembutal. The concentration of oestradiol-17beta in the plasma of Nembutal-treated pro-oestrous rats decreased rapidly during the 24 h after treatment. It is concluded that this decrease in the concentration of oestradiol is due to the inherent ageing of preovulatory follicles manifesting itself when exposure to the ovulatory surge of LH is inhibited.