Transtracheal potential differences and sodium and chloride fluxes in the tracheal epithelium were assessed in normal and bronchitic rats. The results showed that (1) the potential difference was lower in animals with bronchitis; (2) net movement of Na+ and Cl- towards the lumen increased in bronchitis, and (3) the transtracheal potential difference in vivo and in vitro ranged from 4 to 15 mV with the lumen negative to submucosa. It is postulated that changes in the interciliary fluid layer is closely related to the net movement of ions, the number of microvilli-bearing cells and the cholinergic secreting effects of these ions.