A new gonadotrophin releasing hormone antagonist (Nal-Glu) was used during the late follicular phase of the natural cycle in order to prevent spontaneous surges of luteinizing hormone (LH). Eight regularly ovulating women (group 1) received two injections of Nal-Glu (5 mg) administered 48 h apart when plasma oestradiol levels exceeded 125 pg/ml. Human menopausal gonadotrophin (HMG, 225 IU) was administered simultaneously with Nal-Glu and repeated every 12 h thereafter until either a spontaneous LH surge occurred or human chorionic gonadotrophin (HCG, 5000 IU) was administered. HCG was arbitrarily administered 48 h after the second Nal-Glu injection. Six other women (group 2) receiving only HMG served as controls. In seven of the eight women in group 1, LH and progesterone remained low for 96 h following Nal-Glu, i.e. until HCG administration. In the remaining woman in this group, LH started to rise 12 h before HCG injection. In this group, Nal-Glu did not interfere with follicular development or the plasma profile of oestradiol. All women developed one single dominant follicle with the exception of one subject who had already spontaneously developed two dominant follicles prior to administration of Nal-Glu and HMG. In group 2, LH rose spontaneously in all women before the planned HCG injection. The luteal phase was apparently not altered by Nal-Glu. These results suggest that Nal-Glu administration during the late follicular phase of natural cycles supported by HMG, can prevent the spontaneous LH surge while not interfering with follicular growth.(ABSTRACT TRUNCATED AT 250 WORDS)