Hypophyseal portal blood samples were taken from ovariectomized (OVX) ewes given 50 μg estradiol benzoate. This estrogen treatment elicited a biphasic alteration (decrease then increase) in LH secretion. During the negative feedback phase, pulsatile GnRH secretion continued; at this time the interpulse interval for the GnRH pulses (49.5 + 5.7 min, mean + SE, n = 6) was similar to that in 7 control OVX ewes (53.4 + 8.7 min). During the positive feedback phase the GnRH interpulse interval (26.8 + 9.8 min; n = 6) was significantly (P < 0.05) less than in the controls. In 3/7 cases the GnRH pulse frequency in OVX controls was within the range observed for estrogen-treated sheep during the positive feedback phase. These data suggest that, in most cases, the LH surge that can be induced by estrogen in OVX ewes, is associated with an increased GnRH pulse frequency. In some animals the inherent GnRH pulse frequency may already be at a rate that is high enough to permit an LH surge by action of estrogen on the pituitary. In general, the mean concentrations of GnRH in portal blood during the LH surge were higher than those in untreated animals, suggesting an overall increase in GnRH output during the LH surge. Pulsatile GnRH secretion continues throughout the early negative feedback phase, suggesting that the predominant effect of estrogen at this time is at the pituitary level. (Endocrinology116: 2376–2383, 1985)