The study investigated the localisation and immunohistochemistry of neuroendocrine cells (PNECs) and their behaviour in the lungs of Rana ridibunda under experimental conditions and compared with control lungs. Serotonin- and bombesin-immunoreactive (IR) PNECs were observed as solitary cells or clusters of cells at the base of ciliated epithelium on the dilated apical parts of primary septa where incoming air first arrives, and in the respiratory epithelium. Serotonin-IR PNECs were predominantly present in the ciliated epithelium while bombesin-IR PNECs were found in the ciliated and respiratory epithelium. Additionally, bombesin-IR nerve fibres were found in the epithelium and among smooth muscle cells in the connective tissue. In frogs kept in a dry aquarium, serotonin- and bombesin-IR PNECs were characterised by various stages of secretion. Serotonin-IR PNECs released most of their secretory material, while bombesin-IR PNECs were stimulated for secretion. Moreover. bombesin secretions were raised. In conclusion, the cells were identified as members of the pulmonary epithelial endocrine system and can contribute to the pulmonary biology through a paracrine/endocrine pathway in the lungs of R. ridibunda.