Introduction of rams to ovariectomized ewes treated with oestradiol implants (N = 10) increased the frequency of LH pulses from 4 X 8 to 10 X 6 pulses per 12 h. This effect was reflected by increases in mean levels of LH and the basal levels upon which the pulses were superimposed. In ewes that had not been treated with oestradiol (N = 5), there was no significant increase in pulse frequency but mean and basal levels of LH increased slightly after the introduction of rams. In a second experiment, similar effects of the introduction of rams were seen in ovariectomized ewes treated with oestradiol or oestradiol + androstenedione (N = 16), but no significant effects of the rams were observed in untreated ewes (N = 8) or ewes treated only with androstenedione (N = 7). No preovulatory surges of LH were observed in the 30-h period after the introduction of rams. It was concluded that the ram stimulus probably evokes the increase in pulse frequency by inhibiting the negative feedback action of oestradiol, and that the surge normally observed in entire ewes is dependent on the ovarian response to these pulses. However, the observation of responses in some ewes not treated with oestradiol also raises the possibility that the ram stimulus can act directly on the hypothalamic neurones that control the secretion of LH, and that this effect is enhanced in the presence of oestrogen.