This chapter reviews the role of gonadotropin-releasing hormone (GnRH) antagonists in ovulation induction for in vitro fertilization (IVF). Although the purpose of the development of GnRH antagonists was originally a non-steroid contraceptive drug, it was found that GnRH antagonists have potential benefit in assisted reproduction. GnRH antagonists act by immediate suppression of pituitary gonadotropin release and rapid recovery of normal secretion of endogenous luteinizing hormone (LH) and follicle stimulating hormone (FSH). The inhibition of LH secretion is more pronounced than that of FSH, this being most likely due to the different forms of gonadotropin regulation and the prolonged FSH half-life, or the immunoactive and bioactive forms of FSH. The effect of oral contraceptive pills (OCP) for cycle scheduling prior to GnRH antagonist protocol on IVF cycle parameters and pregnancy outcome was studied. All OCP-pretreated cycles required significantly longer stimulation than non-pretreated cycles and higher total dosage of FSH.