The influence on the oestrous cycle and ovulatory response of injecting pregnant mare serum and/or chorionic gonadotrophin has been examined in both the luteal and follicular phases. After a single injection of 1500 i.u. PMSG on Day 13 of the cycle, oestrus was expressed within five to seven days. By contrast, oestrus was delayed until Days 27 to 29 in six gilts injected similarly with PMSG on Day 14. Superovulation was not a characteristic of the response to PMSG injected in the late luteal

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... se. A single treatment with PMSG on Days 17, 18 or 19 slightly shortened the oestrous cycle and provoked superovulation in some of the animals, but treatment in the early follicular phase (Days 15 or 16) was considered the optimum time at which to initiate a superovulatory response. These observations correlate well with the incidence of ovarian compensatory hypertrophy after unilateral ovariectomy performed at different times during the follicular phase. A small proportion of primary oocytes « 10 p. 100) was shed when superovulation followed Day 15 or 16 PMSG, and also after ovulation induced during the luteal phase, whereas up to 80 p. 100 primary oocytes were shed after injection of HCG on Day 17 ; such oocytes invariably underwent polyspermic penetration after artificial insemination. A fairly constant proportion of secondary oocytes (20-35 p. 100) could also be induced to undergo this form of abnormal fertilisation in several experimental situations, suggesting that the susceptibility to polyspermy in apparently mature eggs under specified conditions may be a means of revealing physiological incompetence and thus those eggs destined to undergo early embryonic death. Introduction.