During the last decade, the introduction of GnRH analogs in ovarian stimulati on protocols has greatly improved the outcome of IVF cycles. Through a suppressive effect of hypophyseal secretion, GnRH analogs have allowed to prevent cycle cancellation related to endogenous LH surge and/or premature luteinization. Nevertheless, apart from this beneficial effect, little information has been done about consequences of their administration on residual gonadotroph secretion, ovarian responsiveness to gonadotrophins, quality of oocyte and conceptus, endometrial receptivity. The extreme diversity of long-term and short-term protocols used in clinical practice makes this analysis more difficult. In the first part of this review, the authors examine the several ways of GnRH analogs prescription in order to separate their specific effects on each reproductive function component.