We examined the role of the gonadotropin-suppressing effects of estradiol on the maturation of a single ovulatory follicle in cynomolgus monkeys (Macaca fascicularis) by administering ovine antiestradiol antibodies during the mid through late follicular phase of the menstrual cycle. In each of three control animals, when the ovary containing the maturing follicle was removed during the late follicular phase, histological examination of the remaining ovary 10 days later revealed the presence of a single large maturing follicle. In contrast, in three experimental animals, when estradiol antibodies were infused from days 5 through 10 after unilateral ovariectomy, serum FSH and LH concentrations were elevated above those of control animals, and histological examination of ovaries 10 days after unilateral ovariectomy revealed the presence of two large maturing follicles in the remaining ovary of two animals and four large maturing follicles in the remaining ovary of the third animal. The ability of follicles recruited during passive immunization with estradiol antibodies to respond to exogenous gonadotropin was studied. In three control animals, the maturing follicle was destroyed on day 10 of the follicular phase, and 3 days later, each animal received an ovulatory dose of human CG. None of these control animals produced progesterone. In three experimental animals a continuous infusion of estradiol antibodies was initiated on day 5 of the follicular phase, and the largest antral follicle was destroyed on day 10. Three days thereafter the antibody infusions were terminated and each animal received an ovulatory dose of human CG. Each of these animals produced progesterone despite the destruction of the largest follicle 3 days earlier. These observations demonstrate that estradiol is the principal ovarian modulator of gonadotropin secretion during the follicular phase of the cycle and that interference with the gonadotropin-suppressing actions of estradiol results in continued recruitment and maturation of secondary follicles in the presence of a dominant follicle.