Levocabastine-sensitive neurotensin receptor (NTRL) mRNAs were localized by in situ hybridization in adult and developing mouse brain. NTRL hybridization signal was widely distributed throughout the neuraxis. The highest concentrations of NTRL mRNA were detected in the olfactory system, olfactory tubercle, cerebral and cerebellar cortices, hippocampal formation, and selective hypothalamic nuclei. Moderate to dense hybridization signal was also observed in association with a variety of auditory, visual, and somatosensory relay nuclei, suggesting that the NTRL might be involved in a widespread modulation of primary afferent pathways. Finally a high expression of NTRL was evident in brainstem structures implicated in descending antinociceptive influences (e.g., the periaqueductal gray, nucleus raphe magnus, gigantocellular reticular nucleus, pars alpha, and lateral paragigantocellular nucleus) consistent with the proposed mediation of NT-induced analgesia by the NTRL. Although most of the regions found here to express NTRL mRNA were previously reported to be devoid of mRNA encoding the high affinity NT receptor (NTRH), a few areas (e.g., the anterior olfactory nucleus, medial septum, diagonal band of Broca, reticular thalamic nucleus, suprachiasmatic hypothalamic nucleus, and pontine nucleus) were enriched in both receptor subtypes, suggesting a possible coexpression of these receptors by the same cells. Ontogenic studies revealed that in the mouse brain, NTRL mRNA was detected only from postnatal day 14 and did not reach adulthood concentrations before day 30. In cerebral cortex, the developmental increase in NTRL expression was correlated over time with the decrease in NTRH expression previously documented in the rat, suggesting a progressive takeover of the latter by the former for transduction of the effects of NT in this structure.