The epithelium lining of trachea and lung in rats and mice after administration of 5% dextran iron solution, ferritin and charcoal particles were investigated with the aid of electron microscopy. An enzyme for fluid transport, adenosine triphosphatase, has also been studied by electron microscopy. The microvilli in ciliated cells of the trachea increased in their number after administration of foreign substances, and many micropinoeytosis vesicles take up the ferritin into the ciliated cells but no dextran iron particles were found within the ciliated cell. A large amount of charcoal particles were observed within the ciliated cells with no cilia in cases of inspiration for long hours. All tracers used in this study were found within (he alveolar epithelial cells, so-called dust cells and capillary endothelial cells in lung. ATPase activity could be demonstrated on the surface of the microvilli and cell membrane of ciliated cells but not on cilia.