Neurotensin is localized in discrete populations of neuronal cell bodies with terminals in the hypothalamus and median eminence. High-affinity binding sites for neurotensin have been demonstrated not only in the hypothalamus but also in the pituitary gland. These studies suggest a role for neurotensin in control of hypothalamic-pituitary function. We initially demonstrated that neurotensin could block the release of prolactin in conscious, ovariectomized and male rats after its injection into the third ventricle, whereas intravenous injection of the peptide significantly elevated plasma prolactin and increased prolactin release by pituitaries incubated in vitro. These results suggested that neurotensin had opposite actions on prolactin release, an inhibitory effect at a hypothalamic site and an excitatory one at the pituitary. Further studies employing dopamine receptor blockers and inhibitors of catecholamine synthesis indicated that the action of the peptide to block prolactin release was probably mediated by release of dopamine, which then inhibited prolactin release by the pituitary gland directly. We have evaluated the physiological significance of the peptide in control of prolactin release by intraventricular injection of highly specific antiserum against neurotensin. The antiserum evoked dose-related elevations in plasma prolactin in intact males that were significant but smaller in magnitude than those seen in females, actions opposite to those of the peptide itself, which indicates that the inhibitory action of the peptide within the brain is physiologically significant. Intravenous injection of this antiserum produced a significant suppression of plasma prolactin in females but not males, which indicates that the previously demonstrated stimulatory effect of the peptide on prolactin release by the gland is also physiologically significant because immunoneutralization of the peptide resulted in a decline in plasma prolactin. Our earlier experiments revealed that neurotensin had a dose-related ability to inhibit LH release in ovariectomized and ovariectomized, estrogen progesterone-treated rats. Since it had no effect on the release of LH in vitro, we assigned a hypothalamic site for this action. It appears that this inhibitory effect of the peptide to suppress LH release is also physiologically significant since the intraventricular injection of the antiserum against the peptide produced a dose-related stimulation of LH release in ovariectomized and ovariectomized, estrogen progesterone-blocked rats. The mechanism by which endogenous neurotensin inhibits the release of LHRH has yet to be evaluated.(ABSTRACT TRUNCATED AT 400 WORDS)