The effect of electrical stimulation of the medial preoptic-suprachiasmatic nucleus (MPOA-SCN) region of the forebrain on blood LH levels was studied in ovariectomized, pentobarbital-anesthetized rats. Animals were either not primed with estrogen or previously given 5 micrograms estradiol benzoate (EB)/100 g BW . day for the 2 days before stimulation. Rats were bled continuously (30, 40, or 50 microliter whole blood/5--6 min) through indwelling right atrial cannulae for 1.5 h before stimulation, 1.5 h during stimulation, and up to 1 h afterwards. Whole blood was analyzed for LH by RIA. In ovariectomized, unprimed rats, electrical stimulation of the ventral MPOA consistently increased LH release. In contrast, stimulation of the peri-SCN region (immediately caudal to the MPOA, and lateral and dorsal to, but not within, the SCN), uniformly inhibited pulsatile LH secretion. Activation of the SCN elevated blood LH levels in most unprimed rats tested, but suppression of episodic LH release occasionally occurred. Pretreatment with estrogen resulted in increased LH secretion in response to stimulation of each of these three regions. Estrogen prolonged the LH increase occurring during MPOA stimulation, completely reversed the inhibitory LH response to peri-SCN stimulation, and either reversed any possible inhibitory response to SCN stimulation or greatly increased the magnitude and duration of the increase in blood LH levels produced in the unprimed rat during activation of this nucleus. In summary, the present study indicates that well defined areas in the MPOA-SCN region can have strikingly different effects on LH secretion in the absence of ovarian estrogen and that this steroid is critically important in determining the direction, magnitude, and duration of the LH response to a localized brain stimulus.