The availability of a pure human FSH preparation for studies of the primate ovarian/menstrual cycle permitted novel experiments on gonadotropic stimulation of follicular growth in monkeys. Administration of pure FSH on days 1 through 12 of the menstrual cycle resulted in significant ovarian hyperstimulation, as manifested by the development of multiple (bilateral) ovarian follicles and sustained high serum estradiol levels (approximately 400 pg/ml). In spite of overt follicular development and concurrent increases in serum estradiol, timely LH surges were not elicited. Similarly, during FSH-induced ovarian hyperstimulation, GnRH effects on LH secretion were blunted. Equivalent FSH treatments of long term ovariectomized monkeys had no discernible effects on estrogen-induced LH surges or GnRH responses. Our interpretation is that when supraphysiological FSH levels persist into the late follicular phase, thereby overriding selection of the single dominant follicle of the natural cycle, secretion of an ovarian factor(s) blocks estrogen-induced LH surges.