Substance P (SP)-immunoreactive nerve fibres were searched for at all levels of both fetal and adult human lower respiratory tract. Because the demonstrability of substance P immunoreactivity varies between different animal species, rabbit pulmonary tissue was also subjected to SP immunohistochemistry. Human irises and corneas served as positive human controls. The specimens were taken from 10 human lungs during pulmonary operations. Tracheal tissue was obtained from three patients during bronchoscopy. Five fetal human lungs were examined. Human specimens examined included the trachea, main bronchi, segmental bronchi, and peripheral pulmonary tissue. In addition, the tracheobronchial tissues of four rabbits were studied. SP immunoreaction was demonstrated in formaldehyde-fixed cryostat sections by either the indirect immunofluorescence technique or the peroxidase-antiperoxidase procedure. Both monoclonal and conventional antibodies to SP were tested. In the rabbit SP-immunoreactive nerves were found in both the submucosa and the smooth muscle layer of the main bronchi and trachea. Specimens from human trachea, bronchi, and bronchioli were all negative. Since the SP immunoreaction was easily demonstrated in both human cornea and human iris, it was concluded that there are no SP-immunoreactive nerves in the human pulmonary tissues or that their SP content is very low and below the sensitivity of all the techniques used.