The effect of an intravenous expectorant agent, i.e. Bisolvon, on the secretory activity of the tracheal mucosa of the dog was studied with the scanning electron microscope. A marked increase in the production of tracheal secretion was observed 15 and 30 min following the injection. Both the goblet cells in the epithelium and the seromucinous glands in the tunica propria contributed to the formation of the secretion. 60 min after the injection, the submucosal glands were shrunken or empty. The rapid secretory response to the expectorant, Bisolvon, confirmed the active and important role of the tracheal mucociliary clearance mechanism for the protection of the lower respiratory tract.