Gonadotrophin surge-attenuating factor (GnSAF) is a putative nonsteroidal ovarian factor that attenuates the luteinizing hormone (LH) surge in superovulated women. GnSAF bioactivity was studied during the luteal phase by investigating six normally ovulating women in two cycles – a spontaneous and a follicle-stimulating hormone (FSH)-treated cycle. In both cycles, the pituitary response to an acute intravenous injection (10 μg) of luteinizing-hormone-releasing hormone (LHRH) was investigated in late follicular (follicle size 16 mm), early luteal (day 5 after human chorionic gonadotrophin, hCG), midluteal (day 9 after hCG) and late luteal phase (day 12 or 13 after hCG). FSH was injected daily at the dose of 225 iu on cycle days 2, 3 and 4, and 150 iu thereafter. The increase in LH and FSH (mean ± sem) 30 min after LHRH in the spontaneous cycles decreased significantly from early to late luteal phase and remained unchanged in the FSH-treated cycles. Increases in LH and FSH 30 min after LHRH were significantly attenuated in the FSH-treated compared with the spontaneous cycles in late follicular and luteal phases. Serum oestradiol and progesterone concentrations were significantly higher in the FSH than in the spontaneous cycles only in early, but not in mid- and late luteal phase. The pattern of serum oestradiol and progesterone changes during the luteal phase did not correlate with the increases in LH and FSH 30 min after hCG both in the spontaneous and the FSH cycles. These results suggest that GnSAF bioactivity is high during the luteal phase of superovulated cycles.