Removal of the corpus luteum of the baboon during deturgescence of the perineal sex skin (early stage of the luteal phase) ad-vanced the onset of the next ovarian cycle in 3 out of 4 cases. Progesterone, given as 1 mg every 2 days after removal of the corpus luteum, was unable to reproduce the inhibitory effect of endo-genous progesterone on ovulation in 3 out of 4 baboons, whereas 5 mg progesterone every other day delayed the cycle onset well beyond its usual time. This suggests that withdrawal of circulating progesterone brings about prompt initiation of gonadotrophin secretion and ovarian follicle development. However, removal of the corpus luteum during quiescence of perineal sex skin (middle stage of the luteal phase) did not ad-vance the onset of the next cycle. The implication is either that the corpus luteum in the middle and later stage of the luteal phase is not producing inhibitory levels of progesterone, or that the previous exposure to progesterone has already set in motion a sequence of events which is in-dependent of its continuing influence.
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