Endopeptidase 24.15 (EP 24.15; EC 3.4.24.15), a zinc-metalloendopeptidase purified from rat brain and testes and also present in many other tissues, including the lung, degrades substance P, neurotensin, bradykinin, luteinizing hormone-releasing hormone, and some other bioactive peptides. The enzyme, present both as soluble cytoplasmic and membrane-bound forms, also rapidly converts dynorphin, alpha- and beta-neoendorphin, and some other opioid peptides into their respective enkephalins. In this study, a rabbit antibody to EP 24.15 purified from rat testes was used to study distribution of the enzyme in rat trachea, lung tissue, and alveolar macrophages (AMs) by immunohistochemical techniques. We found intense immunoreactivity to EP 24.15 within the cytoplasm of ciliated epithelial cells of tracheobronchial mucosa extending from trachea to terminal bronchioles. In addition, large myelinated paratracheal and peribronchial nerve fibers showed immunoreactivity. Blood vessels and alveolar lining cells were negative. AMs also showed intense diffuse cytoplasmic immunoreactivity. The findings of EP 24.15 immunoreactivity in airway epithelium, AMs, and paratracheal and peribroncheal nerve fibers suggest that the enzyme may modulate the activities of bioactive peptides within the lung.