Morphological and histochemical studies on APUD[amine precursor uptake decarboxylase]-cells in the lung epithelium are reviewed. Common features of these cells are their capacity to produce and store amine, their argyrophilia and their characteristic cytoplasmic secretory granules. Based on the fine structural morphology of the secretory granules 3 types of cells were observed in the human fetal lung. In the human adult lung, endocrine cells were few in number and widely scattered. Contact between endocrine cells and nerves was never seen. One function of endocrine cells in the human adult lung may be to act as precursor cells for carcinoid tumors and small cell anaplastic carcinomas (the APUDomas of the lung).