The presence of bombesin-like material in nerves and endocrine cells of the gastrointestinal canal, the circulatory system and the lung of the caiman, Caiman crocodylus crocodylus, has been investigated with immunohistochemistry (IHC) and radioimmunoassay (RIA). The effect of bombesin has been studied on the vascularly perfused lung and on isolated strip preparations of the gut wall of the caiman, and on the circulatory system in vivo in both the caiman and the crocodile, Crocodylus porosus. Bombesin-like immunoreactivity is present in water and acid extracts of the muscle layer and the mucosal layer of the gut. IHC demonstrated a dense network of nerve fibres in the sub-mucosa, circular muscle layer and myenteric plexus of the whole gut. A more sparse innervation by single fibres often following septal edges or small vessels is found in the lung. Systemic vessels are innervated by a loose plexus of small fibre bundles. The levels of immunoreactive material in the lung and heart were low. Bombesin is excitatory on strip preparations of the smooth muscle layers of the intestinal wall, and causes an increase in inflow resistance in the perfused lung. In vivo, bombesin increases the blood flow in the left aorta and the coeliac artery.