Sixty-nine women were treated with a long-acting gonadotropin-releasing hormone analogue (GnRHa), D-Trp-6-LH-RH (DTRP6) 3.2 mg (Decapeptyl, Ferring, Kiel, F.R.G.) prior to induction of ovulation for in vitro fertilization (IVF) with human menopausal gonadotropin (hMG; Pergonal; Serono Laboratories, Inc., Randolph MA). Three types of response were noted: Group I (18 patients) responded between days 1-3 of the treatment; Group II (30 patients) responded between days 4-6 of the therapy, and Group III (21 patients) responded after 7 days of treatment. Although a higher number of oocytes were encountered in Group I than in the latter two groups fertilization and clinical pregnancy rates were lower than in Group II (P less than 0.05), respectively, but higher than in Group III (P less than 0.07 and P less than 0.01), respectively. The best results were achieved by Group II patients, while Group III seemed to represent low responders as the number of embryos and pregnancy rate were significantly lower than in the other two groups. It seems that the level of ovarian suppression achieved by GnRH analogue results in a period of refraction of the ovary, and its duration has most probably an effect upon the results of the IVF cycles.