Follicular recruitment takes place under FSH stimulation at the end of the luteal phase and the beginning of the subsequent follicular phase, the so-called luteal-follicular transition (LFT). Inhibin, a known suppressor of FSH, has been implicated in the onset of the rise in FSH levels, as the fall in its immunoreactive plasma levels after the demise of the corpus luteum has been shown to correlate negatively with the increase in FSH. To analyze the role of estradiol (E2), another inhibitor of FSH secretion in the LFT, we designed an experimental paradigm to dissociate the physiological falls in inhibin and E2. This was achieved by extending the duration of luteal plasma E2 levels with transdermal E2 treatment. Untreated ovulatory cycles in seven healthy female volunteers, aged 28-38 yr, were compared with E2-treated cycles in the same subjects, with treatment starting on the 10th day after the LH surge and continuing through the 4th day of the following menses [either 0.2 mg (n = 6; G1) or 0.1 mg (n = 6; G2) E2 daily]. Blood samples were obtained daily from the LH surge until the 11th day of the next cycle. Immunoreactive plasma inhibin levels reached a nadir on day 2 of menses regardless of whether women received E2. Plasma E2 (mean +/- SEM) levels remained within the normal luteal range (220 +/- 51 to 635 +/- 279 pmol/L) until the end of the treatment period in G2 (range, 253 +/- 40 to 382 +/- 62 pmol/L), but not in G1 (range, 598 +/- 195 to 1835 +/- 1259 pmol/L). However, the onset of the FSH rise was clearly delayed, from a mean of 2 days before menstruation in the controls to day 4 of the cycle in G1 and G2. Peak plasma FSH levels were attained within 6 days in the controls and within 2 or 3 days in both treatment periods. Our data suggest that it is the decrease in plasma E2 rather than inhibin that is the triggering signal for the LFT rise in plasma FSH. The exact roles of inhibin and other gonadal proteins (e.g. activins) in follicular recruitment remain to be determined.