OBJECTIVE We studied the time-course of production of gonadotrophin surge attenuating factor (GnSAF) in relation to varying serum concentrations of FSH in women. METHODS Normally cycling women were investigated in four cycles, i.e. a spontaneous cycle treated with placebo (cycle P) and three cycles treated with three different FSH dosages (1 ampoule, cycle 1; 3 ampoules, cycle 3 and 6 ampoules, cycle 6). Placebo or FSH were given as a single i.m. injection on cycle day 2 (0900 h). The response of LH to an i.v. injection of 10 micrograms GnRH (GnSAF bioactivity) was investigated 4, 8, 12 and 24 hours after the injection of placebo or FSH. METHODS Six normally ovulating women with long-standing unexplained infertility were studied. The women were used as their own controls during the cycle treated with placebo. METHODS Pituitary response to GnRH was calculated as the net increase in LH at 30 minutes (delta LH) above the basal value. RESULTS Serum FSH concentrations increased after the injection of FSH in a dose dependent manner. Compared with cycle P, delta LH was significantly attenuated in cycle 3 and cycle 6 at 8, 12 and 24 hours and in cycle 1 at 12 hours after the injection of FSH. Basal concentrations of oestradiol (E2) and LH did not differ significantly among the four cycles at any point except in cycle 6 at 24 hours after the FSH injection when E2 values were significantly higher and LH values significantly lower than in cycle P. CONCLUSIONS These results suggest that GnSAF bioactivity increases significantly as early as 8 hours from a single injection of FSH before any significant increase in serum E2 values. We conclude that in women the effect of FSH on the production of GnSAF in the early follicular phase is concentration dependent.