The topographic and cellular distribution of the neurotensin-hydrolysing neutral metalloendopeptidase 24.16 (EC 3.4.24.16) was examined by light and electron microscopic immunohistochemistry in adult rat mesencephalon. Light microscopic immunoradioautography revealed a ubiquitous distribution of the enzyme throughout the midbrain with a relative enrichment of grey matter areas including the substantia nigra, ventral tegmental area, interfascicular nucleus, interpeduncular nucleus, rostral and caudal linear raphe nuclei, central grey and superficial grey of the superior colliculus. Peroxidase - antiperoxidase immunocytochemistry revealed two distinct cellular patterns of immunostaining: (1) weakly labelled neuronal perikarya more or less uniformly distributed throughout the grey matter, and (2) intensely immunoreactive glial cells heterogeneously distributed across the mesencephalon. Areas exhibiting dense concentrations of endopeptidase 24.16-containing glial cells corresponded to those displaying enhanced immunoreactivity in immunoradioautographs, suggesting that a major proportion of brain endopeptidase 24.16 is associated with glia. Electron microscopic examination of the substantia nigra and ventral tegmental area confirmed the association of the enzyme with a subpopulation of neurons and allowed identification of labelled glial cells as protoplasmic astrocytes. In neurons, endopeptidase 24.16 immunoreactivity was distributed heterogeneously within the cytoplasm of perikarya, dendrites and axons. Reaction product was also characteristically associated with restricted zones of the plasma membrane and underlying neuroplasm. In astrocytes, endopeptidase 24.16 immunostaining was densely and uniformly distributed throughout the cytoplasm of cell bodies and processes. Many of these processes were in direct contact with endopeptidase 24.16-immunopositive neuronal elements. The present results demonstrate that within the midbrain, endopeptidase 24.16 is both intracytoplasmic and membrane-associated in neurons and predominantly intracytoplasmic in glia. The presence of a large number of immunostained elements within areas of the midbrain known to display high levels of neurotensin and/or neurotensin receptors, together with the demonstrated catabolic activity of the enzyme on neurotensin in vitro, is consistent with a role of endopeptidase 24.16 in the functional inactivation of endogenous neurotensin in this region of the brain.