The precise roles of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in the control of preovulatory follicle growth has been re-examined. Suppression of both pulsatile LH secretion and FSH or specific suppression of FSH results in an inhibition of preovulatory follicle growth beyond 2.5 mm dia. Infusion of sheep FSH alone in physiological amounts in the presence of basal, non-pulsatile LH results in the growth of preovulatory follicles. Co-infusion of large amplitude pulses of LH reduced or abolished this effect of FSH. It is suggested that: (1) FSH controls the number of follicles which develop; (2) selection of the large follicle destined to ovulate is directly related to the decline in the plasma concentration of FSH occurring during the period of follicle selection--thus, only the follicle(s) which can withstand this withdrawal of FSH will continue to develop; and (3) pulses of LH may directly affect the action of FSH on the follicle and play an important, hitherto unrecognized role in the selection of the ovulatory follicle by actively inducing atresia.