Polypeptide hormones and growth factors have long been known to internalize into peripheral target cells as a result of their interaction with cell surface receptors. Studies in culture have suggested that certain neuropeptides might undergo a similar type of translocation in neurons. To investigate this possibility in adult mammalian brain, we have examined by confocal laser microscopy the events that follow the binding of fluorescein-tagged derivatives of the tridecapeptide neurotensin to basal forebrain cholinergic cells. Our results demonstrate a selective time- and temperature-dependent internalization of fluo-neurotensin in these cells. This internalization is receptor mediated, proceeds from the entire somatodendritic membrane of the cells, and utilizes endosome-like organelles which are mobilized from dendrites to perikarya and from the periphery of the cell to its perinuclear region. Parallel studies carried out on Sf9 insect cells expressing the rat neurotensin receptor from a recombinant baculovirus indicated that the internalization process involves receptor-ligand complexes and not merely the fluorescent peptide itself. These data suggest that receptor internalization plays a role in neuropeptide signaling in the brain and that it can be harnessed for selective identification of neuropeptide target cells.