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The guinea-pig has an oestrous cycle of 16 to 18 days. Many original observations in reproductive endocrinology such as formation of deciduoma in response to trauma (Loeb, 1907), existence of the oestrous cycle (Stockard & Papanicolaou, 1917) and the ability of the uterus to modify luteal function (Loeb, 1923) involved the guinea-pig as the experimental animal. Several studies have delineated the pattern of progesterone secretion during the oestrous cycle of guinea-pigs (Heap,

Summary. An The corpora lutea were significantly larger in the ante-partum group than in the day-of-calving group. A significantly higher fsh level and lower lh level was found in the pituitaries of the day-of-calving group. Follicular size was significantly lower in the ante-partum and day-of-calving group as compared to both the post-partum groups. Injection of progesterone produced no significant alterations in fsh and lh levels of the pituitary glands. Neither did it depress ovarian follicular growth.

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Prostaglandin F2\g=a\ induced degeneration of embryos in intact pregnant rats but not in ovariectomized rats maintained on progesterone. The pituitary lh stores in the intact, treated rats were significantly higher than those in the pregnant, pseudopregnant or cyclic controls. The prostaglandin failed to inhibit the ovulatory release of lh in the cyclic rat. It is suggested that the luteolytic effect of PGF2\g=a\may be mediated in part by the increased synthesis and secretion of lh.

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