BACKGROUND: The preovulatory rise of progesterone is important for ovulation, but both its regulation and its origin are controversial. Three experiments were performed to determine whether follicular phase progesterone arises from the ovary, the adrenal cortex or both. METHODS: The first study was performed in patients scheduled for assisted reproduction, who received a long-acting GnRH agonist either during intake of an oral contraceptive or during the luteal phase of an otherwise untreated menstrual cycle. The second study was also performed during down-regulation with a GnRH agonist: some patients with elevated progesterone levels received dexamethasone (DXM). Others with similarly elevated basal progesterone levels and those with low progesterone levels were not treated with DXM and served as controls. Finally, adrenocorticotrophic hormone (ACTH) tests were performed in normocyclic volunteers both during early and late follicular phase and during intake of a contraceptive pill. RESULTS: During the suppression of endogenous gonadotrophin secretion progesterone levels rose after the administration of ACTH, but not of GnRH. DXM did not prevent the preovulatory rise of the serum progesterone concentration. The ACTH-stimulated concentration of progesterone and of 17α-hydroxyprogesterone were significantly reduced during intake of ethinyl estradiol. CONCLUSIONS: Progesterone arises in the adrenal cortex during most of the follicular phase, whereby its function is modulated by an unknown ovarian factor, which is suppressed by ethinyl estradiol. The source of progesterone shifts towards the ovaries prior to ovulatio
