The dynamics of ovarian and pituitary hormone changes during the midcycle period were evaluated. Changes in hormone levels were determined at 2-h intervals for 5 consecutive days during the periovulatory phase of the cycle in five women.

During the 50 h preceding the onset of the surge, the rates of increments for estradiol (E₂), progesterone (P₄), and LH were similar, with doubling times of 57–61 h. The onset of LH and FSH surges was found to occur abruptly (LH doubled within 2 h). They were temporally associated with the attainment of peak E₂ levels and occurred 12 h after the initiation of a rapid rise of P₄. The mean duration of the surge was 48 h, with a rapidly ascending limb (doubling time, 5.2 h) lasting 14 h accompanied by a rapid decline of E₂ and a continued rise of P₄. The surge was followed by a peak plateau of gonadotropin levels lasting for 14 h and a transient leveling of P₄. The longer descending limb (half-time, 9.6 h), lasting for 20 h, was associated with a second rapid rise of P₄, beginning 36 h after surge onset or 12 h before termination of the surge. By using the onset of the LH surge as a reference point, our data provide a relatively precise picture of the hormonal changes preceding the onset of the gonadotropin surge and the temporal relationship between the multiphasic P₄ rise and pituitary-ovarian function.