In-vivo and in-vitro studies have provided evidence that a non-steroidal ovarian factor, called gonadotrophin surge attenuating factor (GnSAF), attenuates the endogenous LH surge in superovulated women. To study the mechanism of action of GnSAF, the LH response to two i.v. pulses of GnRH (10 micrograms each, 2 h apart) was investigated in eight normally ovulating women during the late follicular phase of a spontaneous and an FSH superovulated cycle. The maximal LH increase in response to the first pulse (initial release) was considered as representing the acutely releasable pool and the delta LH area under the whole curve (integrated response) the reserve pool of LH. Both the initial release and the integrated response to GnRH were markedly attenuated in the FSH as compared to the spontaneous cycles. The response to the second pulse was significantly greater than the response to the first pulse (self-priming effect of GnRH) in both the spontaneous and the FSH cycles. However, in the FSH cycles the self-priming effect of GnRH was markedly reduced as compared to the spontaneous cycles. We conclude that during superovulation induction in women the two pools of pituitary LH are markedly attenuated. It is suggested that GnSAF attenuates both the GnRH-induced initial release of LH and the self-priming effect of GnRH on the pituitary.