Neurotensin immunoreactive perikarya, fibers and nerve terminals, visualized by the indirect immunohistofluorescent method in colchicine-pretreated animals, are localized in many discrete regions of the rat brain stem. Cell body groups are found in the inner aspect of the substantia gelatinosa of the caudal trigeminal nuclear complex, the nucleus of the solitary tract, the parabrachial nuclei, the locus coeruleus, the dorsal raphé nucleus, the periaqueductal gray matter, and the ventral tegmental area of Tsai. These areas of cell body density are accompanied by concentrations of fibers and terminals, while the occasional positive perikaryon noted in the dorsal cochlear nucleus is accompanied by only sparse fluorescent fiber/terminal patterns. Other brain stem regions, such as the floor of the fourth ventricle and aspects of the caudal ventrolateral reticular formation, possess substantial numbers of fibers and terminals that are not accompanied by cell bodies. Many aspects of this distribution coincide with the brain stem distribution of the enkephalin pentapeptides, though significant differences in localization are also evident. Interactions of neurotensin with other neurotransmitter candidates are also suggested by its presence in areas enriched in norepinephrine, dopamine, serotonin, and substance P. Certain neurotensin localizations suggest an association of the peptide with functional brain systems preferentially involving these regions. In particular periaqueductal gray and substantia gelatinosa neurotensin synapses are plausible sites for the analgesia elicited after intercisternal injection of low doses of neurotensin.