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Summary. Groups of rats ovariectomized 5 weeks previously were injected daily for 6 days with oestradiol and/or progesterone. Although the dose of oestradiol used was adequate to suppress peripheral serum LH values, none of the treatments significantly altered the peripheral serum levels of prostaglandin F. Caldwell, Tillson, Brock & Speroff (1972) showed that in ovariectomized ewes injected with progesterone every 2 days over a 12-day period, followed 2 days later by a single injection of oestradiol, the prostaglandin F level in peripheral plasma rose significantly about 6 hr after the oestrogen treatment. Levels of prostaglandin F had returned to those seen before the oestrogen injection by 18 to 24 hr after the oestrogen treatment. In some ovariectomized animals, a slow rise of prostaglandin F was observed during the period of progesterone injections but a peak level was seen only after oestrogen stimula¬ tion. In addition, Blatchley, Donovan, Poyser, Horton, Thompson & Los (1971) and Blatchley, Donovan, Horton & Poyser (1972) have reported that daily injection of 10 µg oestradiol benzoate on Days 4 to 6 of the oestrous cycle of the guinea-pig caused elevated levels of prostaglandin F in the utero- ovarian venous plasma on Day 7. It has been postulated by Caldwell et al. (1972) that the rising levels of oestrogen on Day 13 of the oestrous cycle of the ewe are responsible for the production of prostaglandin F in the endometrium. This prostaglandin reaches the ovarian artery by the counter-current mechanism postulated by McCracken, Baird & Goding (1971) and causes a rapid fall in progesterone secretion on Days 13 to 14 of the cycle and demise of the corpus luteum. We have therefore studied the effects of progesterone and oestrogen on the prostaglandin F levels in ovariectomized rats, and, for reference purposes concerning the effectiveness of the experimental treatments, have included measurements of LH levels in the same samples. Both progesterone and oestradiol have been found to inhibit or stimulate the secretion of LH depending on the dose, the time following ovariectomy, the time of day of steroid ad-

EFFECTS OF EXOGENOUS OESTRADIOL AND PROGESTERONE ON SERUM LEVELS OF PROSTAGLANDIN F AND LUTEINIZING HORMONE IN CHRONICALLY OVARIECTOMIZED RATS