Very small electrolytic lesions were made over the anterior or posterior portion of the optic chiasm in mature female rats showing normal estrous cycles. Lesions over the posterior portion of chiasm destroyed the suprachiasmatic nucleus of the hypothalamus (SCN) while the anterior lesions destroyed a small neural structure, designated as the medial preoptic nucleus (MPN). Both lesions were effective in inducing persistent vaginal estrus, but when animals were ovariectomized and treated with exogenous estrogen and progesterone it was found that lesions including the MPN alone, but not the SCN alone, eliminated the positive feedback effects of this steroid regimen on LH [luteinizing hormone] release.