Biochemical and anatomical data are reported which demonstrate for the first time the existence of a retrograde axonal transport process for a neuropeptide, neurotensin, in rat brain. Neurotensin receptors are mainly located in the striatum on nerve terminals of the nigrostriatal dopaminergic pathway. Thus, the association of specific neurotensin receptors on a well defined pathway provides an excellent model to investigate the existence of such a process. Two hours after the intrastriatal injection of iodinated neurotensin, radioactivity started to accumulate in the ipsilateral substantia nigra. The levels were maximal during the fourth hour. The appearance of this labelling was prevented by injection of a large excess of unlabelled neurotensin or of neurotensin 8-13, an active neurotensin fragment, but not by neurotensin 1-8 which had no affinity for neurotensin receptors. These results suggest that the appearance of radioactivity in the ipsilateral substantia nigra was dependent on the initial binding of this peptide to its receptors in the striatum. HPLC studies demonstrated that the radioactivity found in the substantia nigra corresponded to intact neurotensin and to degradation products of this peptide. Moreover, it has been shown that this retrograde transport was microtubule-dependent and occurred in dopaminergic nigrostriatal neurons. Light and electron microscopic data confirmed and extended the present results. Four and a half hours after intrastriatal injection of iodinated neurotensin, silver grains were mainly detected in dopaminergic perikarya of the substantia nigra pars compacta. The vast majority were associated with neuronal elements and their localization within cell bodies suggests that retrogradely transported neurotensin may be processed along a variety of intracellular pathways including those mediating recycling in the rough endoplasmic reticulum and degradation in lysosomes. However, the presence of silver grains over the nucleus, as well as the increase in tyrosine-hydroxylase mRNA expression in the ipsilateral substantia nigra 4 hr after intrastriatal injection of neurotensin support the concept that neurotensin alone, or associated with its receptor, might be involved in the regulation of gene expression. Finally, we have demonstrated that in old rats the quantity of retrogradely transported neurotensin was significantly decreased as compared to that observed in young adult rats. This retrograde axonal transport of a neuropeptide may represent, as already suggested for growth factors, an important dynamic process conveying information from nerve terminals to the cell body.