The serotonergic system, because of very diffuse projections throughout the central nervous system, has been implicated in numerous functions including nociception, analgesia, sleep-wakefulness and autonomic regulation. Despite an abundant literature indicating the presence of neurotensin-containing (neurotensinergic) neurons, fibres and terminals in most areas containing serotonergic neurons, little is known about the possible relationship between serotonergic and neurotensinergic systems. The purpose of this review is (i) to summarize current knowledge on the anatomical relation between neurotensinergic and serotonergic system, (ii) to summarize current knowledge on the action of neurotensin on serotonergic neurons and (iii) to discuss the possible physiological relevance of this action. Neurotensin-containing cell bodies can be found in the most rostral raphe nuclei. There are neurotensin-containing fibres and terminals in all raphe nuclei. Raphe nuclei have also been shown to contain neurotensin-receptor binding sites. In the dorsal raphe nucleus, neurotensin induces a concentration-dependent increase in the firing rate of a subpopulation of serotonergic neurons. The neurotensin-induced excitation, which is selectively blocked by the non-peptide neurotensin receptor antagonist SR 48692, is observed mainly in the ventral part of the nucleus. Most serotonergic neurons show marked desensitization to neurotensin, even at low concentrations. In intracellular experiments, neurotensin induces an inward current, associated in some cases with a decrease in apparent input conductance, which is occluded by supramaximal concentrations of the alpha 1-adrenoceptor agonist phenylephrine. In rare cases, neurotensin induces an excitation of GABAergic or glutamatergic neurons. Since the neurotensinergic system has also been implicated in nociception, analgesia, sleep-wakefulness, and autonomic regulation, the review discusses the possibility that part of this regulation could involve the activation of the serotonergic system.