The basal cell in airway epithelium plays a major role in attachment of ciliated and nonciliated columnar cells to the basal lamina. As the airway grows in diameter and the columnar epithelium in height, the number of basal cells and the amount of tonofilaments (cytokeratin filaments) and anchoring junctions increase. In this way they maintain a constant attachment strength between the increased volume of the epithelium and the basal lamina. The purpose of this study was to determine which cytokeratins (CKs) are expressed in growing basal cells of the rat and demonstrate where they are localized in the cytoskeleton. Sprague Dawley rats 10, 30 and 90 days of age were used in this study. For light microscopy, tracheal samples were fixed in 95% alcohol or 4% formalin for 2 hr and then embedded in paraffin. For electron microscopy, the tracheal samples were placed in 20 mM EDTA in HBSS media minus Ca++ and Mg++ at pH 7.4 for 60 min to permeabilize the cells and expose the intracellular structures. Antibodies to cytokeratins 7, 8, 10, 13 and 18 did not react to basal cells at any age studied. Antibodies to CKs 5 + 8, 14, 16 + 13, and 19 gave a positive reaction with basal cells at each age studied. Immunogold particles representing antibodies to CK 14 were heavily distributed over intermediate filaments making up the cytoskeleton. Both CK 16 + 13 and 19 were also over intermediate filaments but at a much lower density.(ABSTRACT TRUNCATED AT 250 WORDS)