The indirect immunoperoxidase (PAP) method was used on chicken lung specimens from embryos ranging in age from 6 days to hatching, chicks and adult chickens of up to 6 months. The ontogenesis and distribution of neurons and paraneurons containing immunoreactivities for serotonin (5HT), bombesin, vasoactive intestinal polypeptide (VIP), substance P (SP) and galanin were investigated. Serotonin-immunoreactive paraneurons were first detected in the pulmonary mesenchyma of 8-day-old embryos, while in the 12-day-old embryos the following neurons and paraneurons were first detected in their respective locations: serotonin-immunoreactive paraneurons in the bronchial epithelium; VIP- and galanin-immunoreactive ganglionic cells and SP-immunoreactive nerve fibres in the intrapulmonary ganglia. At hatching, serotonin-immunoreactive paraneurons in the epithelium of the air capillaries and air sacs, and bombesin-immunoreactive paraneurons in the epithelium of the primary bronchus, VIP-, galanin- and SP-immunoreactive nerve fibres in the lamina propria of the primary and secondary bronchi and in the pulmonary septa could also be shown. Some serotonin-immunoreactive small paraneurons were also found in the intrapulmonary ganglia. In the adult specimens, VIP-, galanin- and SP-immunoreactive nerve fibre networks were observed throughout the primary bronchus wall and in the lung septa. In intrapulmonary ganglia, VIP- and galanin-immunoreactive neurons and serotonin-immunoreactive small paraneurons could be more numerously demonstrated. Moreover, bombesin paraneurons occurred in the epithelium of primary and secondary bronchi, and serotonin-immunoreactive paraneurons were found in the epithelia of the bronchi and air sacs and in some pluricellular bodies in the lamina propria of the air sac ostia.