Active ion transport across the surface epithelium of dog trachea results in fluid transport. The direction and amount of fluid transported may depend on a balance between active chloride secretion and active sodium absorption. In contrast, fluid movement across submucosal gland epithelium does not seem to depend on active salt transport. Instead, water may be drawn into the duct lumen by simple osmosis after the release of the osmotically active contents of secretory granules. Cell culture may provide a means of studying the secretory properties of the individual cell types of airway epithelia. We have produced primary monolayer cultures of cells from dog tracheal epithelium which retain the differentiated salt and water transport of the original tissue.