Rats were hypophysectomized at oestrus, at days 1, 2 and 3 of dioestrus or at pro-oestrus. They were injected with various doses of PMS only, or with combinations of PMS and HCG, in such a way that fairly constant blood levels could be expected, and killed 24 h later. It was found that the minimal doses of substitutional gonadotrophins, required to maintain normal follicular growth, were considerably higher during the period from pro-oestrus to oestrus (when a new generation of follicles begin to develop) than during all the other periods investigated. This peak requirement primarily concerned PMS. The minimal requirements during other periods showed only minor variations. The maximal doses which still caused normal follicular growth were higher during the period from dioestrus −1 to −3 than during the period from oestrus to dioestrus −1 and the period from dioestrus −3 to pro-oestrus. The data suggest that during pro-oestrus very high amounts of FSH are required to recruite a new crop of follicles, whereas during the other phases, low levels of both FSH and LH are generally needed for normal follicular growth.
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R. Welschen (1973) studied this question.