Abstract
It is now well established that progesterone can both facilitate and inhibit ovulation in the rat. In the adult female rat these responses are dependent on the stage of the oestrous cycle as was shown by Everett (1948) and recently confirmed by Zeilmaker (1966). A hypothalamic site of action for progesterone in the facilitation of ovulation was demonstrated by Barraclough, Yrarrazaval & Hatton (1964). On the other hand, ever since Hohlweg (1934) observed the induction of corpus luteum formation by oestrogen in prepubertal female rats, a close connexion between the preovulatory rise of oestrogen and ovulation-inducing gonadotrophin release was suggested by many investigators, and Döcke & Dörner (1965) concluded that oestrogen may increase the sensitivity of the hypophysis to the stimulating effect of the hypothalamic gonadotrophin-releasing factor(s). The present study was made to see whether a synergism between oestrogen and progesterone in the induction of ovulation can be shown in

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