There have been a number of studies of GnRH secretion into the hypophysial portal blood at the time of the estrogen-induced LH surge, but the exact pattern of secretion at the onset of the positive feedback event remains a point of some dispute. In the present study, GnRH concentrations in portal plasma and jugular venous LH concentrations were measured in samples taken at 2.5-min intervals from five ovariectomized control ewes and seven ovariectomized ewes that were treated with 50 μg estradiol benzoate (im) to cause a LH surge. The frequency of GnRH pulses was calculated using a modification of previous criteria because of the frequent sampling. It was found that GnRH pulse frequency increased during the LH surge, being greatest during the “leading edge” of the surge. Perhaps more importantly, the data show that variable patterns of GnRH secretion occur at the start of the LH surge and during the surge, suggesting that no single pattern can be identified as an initiation signal of the positive feedback event. These data are consistent with other results that have been obtained in this laboratory in cyclic animals.