The effect of stimulation of the bed nucleus of the stria terminalis (BNST) on ovulation and LH release was studied in unanesthetized, freely moving rats. Electrochemical stimulation (anodic d.c, 100 µA/30 sec) was applied at i 1.30 h through chronically implanted monopolar, stainless steel electrodes. Serial blood samples were obtained by way of a plastic cannula implanted in the jugular vein. Stimulation on the day of proestrus prevented ovulation and the preovulatory discharge of LH when the stimulus was applied to the lateral part of the BNST but advanced the surge of LH in those rats stimulated in the medial part of the nucleus. In ovariectomized estrogen-primed rats stimulation in the medial part of the BNST produced LH release but no effect was seen when the stimulus was applied in the lateral part. It is concluded that the BNST is part of a dual system with antagonistic effects on LH secretion