The epithelium of the conducting airways of the mouse consists of a single layer of cells. The number, type and form of these cells have been investigated at five airway levels from the trachea to the distal conducting bronchi with both light and electron microscopes. Contrary to what is found in other species, the majority (50-60%) of cells in the murine airway epithelium are Clara cells. Mucus-producing tissue was infrequent throughout the airways, though epithelial mucous cells occurred in increased numbers at the carina and in the primary bronchus. No mucous or serous cells or submucosal glands were seen in intralobular airways. On a morphological basis, three distinct forms of Clara cell were recognized. On occasion, cells were observed which were apparently transitional types between these and also between Clara cells and mucous or ciliated cells. It is suggested that the'transforming'cells may indicate a role for the Clara cell as a developmental cell involved in the epithelial cell turnover. Evidence is also provided that Clara cells may undergo both apocrine and merocrine secretion and, it is argued that the latter may be of a PAS+ ve material. Free nerve endings were not seen in the epithelium. This may be related to athe restricted ability of mice to cough. It is suggested that the lack of mucus-producing tissue and of cough reflex may be due to the small diameter of the mouse airways.