We and other investigators have demonstrated the stimulation of luteinizing hormone secretion following the microinjection of neurotensin (NT) into the medial preoptic area in female rats. The present study electrophysiologically examined the action of the peptide in the same brain region. Microelectrophoresis of NT produced either excitation or inhibition in 24% of the medial preoptic neurons tested in oestrogen-primed and unprimed ovariectomized animals. Effective blockade of NT-induced excitatory and inhibitor unit responses by means of the simultaneous application of an antiserum to NT (anti-NT) indicated that they were produced by specific actions of the peptide. In addition, anti-NT by itself altered the spontaneous firing rate in some NT-responsive cells. These findings are in accord with the hypothesis that the preoptic area is a site of the action of NT in the female rat.The medial preoptic area comprises neurons containing luteinizing hormone-releasing hormone (LHRH) (1) and is widely accepted as a regulatory center for luteinizing hormonc (LH) secretion in the female rat. Evidence has been accumulated showing that neurotensin (NT), a tridecapeptide first identified in the bovine hypothalamus (2), is one of the neuromodulators which regulate the activity of this center to affect the hormone secretion. Microinjection of NT into the medial preoptic area significantly enhanced LH secretion in anesthetized female rats (3) and in conscious ovariectomized oestrogen-primed rats (4). The peptide therefore probably acts directly in the preoptic and/or adjacent areas which contain NT-related neural elements (3,5). A recent finding that a series of basal forebrain injections with an antiserum to NT reduced the magnitude of the steroid-induced surge of LH in the ovariectomized rat (6) further supports this hypothesis. The aim of the present study was to electrophysiologically examine the relevance of the preoptic area as a site of NT action in female rats. The specificity of the unit response was verified with a specific antiserum to NT.