The effect of hypothalamic lesions on estradiol-induced changes in plasma luteinizing hormone (LH) concentration was studied in ovariectomized ewes. The ewes had stainless steel electrodes chronically implanted in the hypothalamus (HYP), and electrolytic lesions were made within 1 h after the injection of 50 microgram estradial benzoate (EB) i.m. Blood samples were collected at 2--6 h intervals for 30 h after EB, and the plasma was subsequently assayed for LH. None of the lesions produced had any effect on the normal immediate (within 12 h after injection) inhibitory effect of estrogen on LH release. However basal hyothalamic lesions, ranging from the suprachiamatic region back over the optic chiasma down to include the anterior tuberal region, effectively inhibited the normal stimulatory effect of estrogen (12--30 h after injection) on LH release. The results are consistent with the interpretation that, in the normal biphasic LH response to injected estrogen in the ewe, the early inhibition of LH release results from direct inhibition of pituitary sensitivity to endogenous LH-releasing factor (LRF). On the other hand the estrogen-induced LH release which occurs subsequent to this inhibitory phase is principally the result of increased LRF secretion by the HYP.