Basal forebrain cholinergic neurons project to the hippocampus and cerebral cortex where they play an important role in cortical activation, attention, and memory. These neurons have been shown to express functional neurotensin receptors and to receive a dense neurotensinergic innervation. In the present study, we investigated the origin of this innervation by combining retrograde transport of cholera toxin with immunohistochemical detection of neurotensin. After injection of cholera toxin in the anterior substantia innominata and diagonal band of Broca, retrogradely labelled cells were widely distributed throughout forebrain limbic structures. Only a small proportion of these cells, located (by decreasing order of frequency) in the lateral septum, medial preoptic area, rostral hypothalamus, nucleus accumbens, and rostral basal forebrain, were dually labelled for neurotensin. After injection of cholera toxin in the posterior substantia innominata and magnocellular preoptic nucleus, retrogradely labelled cells were detected throughout the limbic forebrain and pontomesencephalic tegmentum. Here again, only a small proportion of these cells, located (by decreasing order of frequency) in the nucleus accumbens, lateral septum, rostral basal forebrain, hypothalamus, bed nucleus of the stria terminalis, supramammilliary nucleus, ventral tegmental area, and raphe complex co-localized neurotensin. In view of the burst generating properties of neurotensin on basal forebrain cholinergic neurons, our results suggest that neurotensin projections may be part of the septo-hippocampo-septal loop regulating hippocampal theta activity. More caudally, neurotensin axons originating from the lateral hypothalamus and pontomesencephalic tegmentum may contribute to the contingent of ascending reticular formation fibers involved in the regulation of the sleep-wake cycle.