The gonadotrophin-releasing hormone (GnRH) antagonists Cetrorelix and Ganirelix have been used in recent years in clinical studies to prove that these compounds reliably prevent the onset of premature luteinizing hormone (LH) surges during ovarian stimulation. Cetrorelix has been applied in single and multiple dose protocols, while Ganirelix has until now only been used in the multiple dose protocol. In the latter protocol, ovarian stimulation is started on day 2 or 3 of the spontaneous cycle with human menopausal gonadotrophin or recombinant follicle stimulating hormone. Daily administration of the GnRH antagonist at its minimum effective dose (0.25 mg/day s.c.) occurs from the sixth day of stimulation onwards until ovulation induction by human chorionic gonadotrophin. In the single dose protocol, 3 mg of the GnRH antagonist Cetrorelix was injected on day 8 of the stimulation cycle. Both protocols have been proven to be safe and effective. Fertilization rates of >60% in in-vitro fertilization and >70% in intracytoplasmic sperm injection, as well as clinical pregnancy rates of approximately 30% per transfer, sound most promising. The incidence of a premature LH surge is below 2%. The incidence of severe ovarian hyperstimulation syndrome seems to be lower under antagonist treatment than in the long agonistic protocol. Treatment time is significantly shortened.