In this study the effect of multiple follicular development on the endocrinology of the menstrual cycle was investigated. Normally ovulating women were studied during control spontaneous cycles and cycles superovulated with different treatment regimes. Although clomiphene induced an increase in basal secretion of both FSH and LH, superovulation induction with human gonadotrophins resulted in a marked reduction in basal gonadotrophin secretion. It is suggested that basal secretion of FSH and LH is regulated by two separate mechanisms. The occurrence of an endogenous LH surge in superovulated cycles is dependent on the treatment regimen. Both clomiphene and unspecified ovarian factors are important regulators. When in these cycles an LH surge occurs, it is markedly attenuated both in amplitude and duration. The attenuation is due to ovarian factors different from oestradiol and progesterone. Corpus luteum function in superovulated cycles is disrupted and this seems to be related to the marked reduction in pituitary LH secretion. Ovarian hyperstimulation is a potent stimulus of prolactin secretion and the effect seems to be mediated by oestradiol. Folliculogenesis in superovulated cycles is FSH dependent, however follicular growth rate is similar to that in spontaneous cycles. It is concluded that ovarian hyperstimulation in normally cycling women induces marked changes in endogenous gonadotrophin secretion characterized by an augmentation of the negative and an attenutation of the positive feedback mechanism. Both these effects are mediated by unspecified ovarian factors.