Inhibin-like activity as well as estradiol-17 beta and progesterone content were assayed in human follicular fluid obtained at different stages of the periovulatory period. Between the start of the midcycle estrogen surge in plasma and the subsequent gonadotrophin peak, follicular fluid was found to contain similar levels of estradiol-17 beta and progesterone (8.2 pmol/microliter) and to induce a marked (44%) inhibition of gonadotrophin-releasing hormone-stimulated release of FSH. The midcycle gonadotrophin peak was characterized by a drop in estradiol-17 beta levels and a tripling in progesterone levels, whereas inhibin-like activity fell to almost half the previous value.