The luteolytic properties of prostaglandin F2\g=a\(PGF2\g=a\) were studied in experiments on sheep with ovarian autotransplants. PGF2\g=a\ was infused at the rate of 40, 10 and 2 \g=m\g/h for 4\m=.\0, 7\m=.\0and 9\m=.\5-18\m=.\0h respectively. At rates of 40 and 10 \g=m\g/h,progesterone secretion decreased to 50% of the control levels in 2\m=.\6\ m=+-\1\m=.\4h and fell to 8-30 \g=m\g/h within 36-52 h from the start of infusion. The onset of oestrus occurred 42-66 h after the infusions had started and a typical pre-ovulatory surge of luteinizing hormone (LH) was observed 52-66 h after infusing PGF2\g=a\. On occasions, this surge was accompanied by a brief peak of oestradiol-17\g=b\ secretion and shortly thereafter ovarian steroidogenesis decreased to very low levels.In four sheep infused at a rate of 2 \g=m\g/h,peripheral progesterone values decreased gradually and only one animal returned to oestrus. It would seem that 2 \g=m\g/h is close to the minimum intra-arterial dose to cause luteolysis in the sheep.

A synthetic decapeptide (Gn-RH) structurally identical to ‘ovine LH-releasing hormone’ was given by intra-carotid infusions (0, 0.5 and 1.5 µg/h for 3 h) to two groups of ewes late in the anoestrous season. One group received pretreatment with intra-venous progesterone (500 µg/h for 24 h) and the other no such treatment. Gn-RH was found to be a highly potent LH releasing agent capable of inducing ovulation. Physiological amounts of pro gesterone were incapable of preventing Gn-RH-induced pituitary LH release. It is con cluded that the blockade of LH release during the luteal phase or after progesterone administration may be due to steroid action at the level of the hypothalamus rather than at the pituitary.

Part. I: Prostaglandin F_2Α (PGF_2Α) was shown to be ‘the’ luteolysin in the ewe on the following criteria: (1) Hysterectomy and/or separation of the uterine horn from the ovary bearing a corpus luteum led to prolongation of the oestrous cycle, (2) PGF_2Α shortened the cycle when administered in the midluteal phase, (3) PGF_2Α was identified in uterine venous blood, (4) a mechanism for the transfer of PGF_2Α from the uterine vein to the ovarian artery was shown to exist, (5) the quantitative aspects of the secretion transfer mechanism and luteolytic potency of PGF_2Α were adequate to account for the observed phenomena. Part II: These criteria were applied for evaluation of PGF_2Α as ‘the’ luteolysin in the following species: cattle, goat, horse, pig, guinea pig, rat, hamster, rabbit, monkey, human. On present knowledge, there appeared to be a variation between species from those in which PGF_2Α was probably concerned in luteolysis and those in which the evidence was against such a role. Part III: PGF_2Α was shown to be luteolytic when given by continuous infusion for 3–6 h into the lumen of the uterus in cattle as well as sheep. The minimum effective dose was of the order of 7 g/kg. The application of this finding to artificial insemination programmes was discussed.

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