Summary. Plasma progesterone determinations were carried out on blood samples collected daily from Clun Forest ewes during the normal oestrous cycle and also after administration of LH-releasing hormone (LH-RH) during seasonal anoestrus.Levels of plasma progesterone at oestrus ranged from 0\m=.\1to 0\m=.\5 ng/ml and luteal phase levels from 3 to 6 ng/ml. Levels found during seasonal anoestrus were within the range of those observed at oestrus. Following treatment with LH-RH, there was an increase in the plasma LH level in all cases and ovulation occurred in twenty-three out of twenty-seven treated ewes. In the animals which ovulated, the plasma progesterone concentration either remained basal (eighteen animals) or rose to a lower level (<2 ng/ml) than that found during the luteal phase of the cycle.

The status of the anterior pituitary-ovarian system of the sow was examined during lactational anoestrus and after weaning up to the occurrence of the first oestrus after weaning. The ovaries and uteri were examined and the follicle-stimulating hormone (FSH) and luteinizing hormone (LH) contents of the anterior pituitary glands were measured.Suppression of follicular growth and uterine development was apparent during lactation. This was relieved by weaning, when follicle size increased and oestrus and ovulation occurred within 6 days.A high level of pituitary FSH was found during lactation which was maintained after weaning until oestrus occurred, when the level fell. The level of LH was low during lactation, rose after weaning and fell when oestrus occurred.Ovariectomy of sows during lactation did not affect the growth of the young. While ovariectomy during the oestrous cycle produced the expected uterine atrophy, ovariectomy during lactation did not result in any further decline in uterine weight or length, suggesting that oestrogen secretion is at a very low level during late lactation. Pituitary LH content rose as a result of ovariectomy performed during the oestrous cycle but failed to do so as a result of ovariectomy performed during lactation, thus demonstrating inability of the lactating sow to increase LH synthesis under circumstances when the cycling sow is capable of doing so.From these results it is postulated that (a) a considerable release of both FSH and LH occurs at about the time of the after-weaning oestrus, similar to that which is believed to occur at cyclic oestrus, (b) failure primarily of release of FSH and of synthesis of LH occurs during lactation which accounts for suppression of follicle growth, anoestrus and anovulation during lactation in the sow.

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It has been shown that administration of a single intravenous injection of 150 or 300 \g=m\gsynthetic LH-releasing hormone (LH-RH) to seasonally anoestrous Clun Forest ewes induced LH release in all animals and ovulation in the

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