Epithelial thickness, depth of the ciliary layer and concentration of cells has been estimated at 5 levels of the rat airway (3 extrapulmonary and 2 intrapulmonary) and the ultrastructure of the cells described. Extrapulmonary airways have a pseudostratified epithelium, intrapulmonary airways a simple one. The epithelium thins progressively from upper to lower trachea while the epithelium of the lower trachea is thicker than at more peripheral airway levels, all of which are similar. The depth of the ciliary layer decreases peripherally. By electron microscopy 10 cell types could be identified - 8 epithelial and 2 'migratory'. A cell type not described previously has been found that resembles the serous cells of the submucosal glands. The salient features of each cell type are described. The upper trachea has a concentration of cells significantly higher than elsewhere, while the remaining four airway levels have about 230 cells per 10 high power fields (1-8 mm length epithelium). Ciliated cells are sparse proximally and increase in number progressively toward the periphery, while basal and migratory cells are most frequent proximally. At all levels of the airway, between 40% and 50% of cells are non-ciliated. In the upper trachea the most frequent cell type was the 'intermediate', elsewhere the 'epithelial serous' cell. Goblet cells were few at all levels, amounting to less than 1% of the total counted. The Clara cell was not restricted to the terminal bronchioles but was found as far proximally as the hilum.