Effects of microinjection of neurotensin (NT) into the medial preoptic area on luteinizing hormone (LH) release were examined in unanesthetized ovariectomized rats with or without estrogen priming. Microinjection of NT (100 ng) into the medial preoptic area of ovariectomized estrogen-primed rats significantly facilitated the circadian afternoon rise of LH secretion as compared to the hormone values in control animals microinjected with physiological saline. The timing of the afternoon LH rise was not altered by the peptide. In ovariectomized estrogen-unprimed rats, on the other hand, NT did not affect the existing pulsatile LH secretion nor induced an afternoon rise of the hormone secretion. These results show that NT can stimulate the preoptic LH release mechanism which undergoes the circadian clock under the influence of estrogen.