The influence of five inhalation anesthetics on lung function and bronchial caliber was determined in dogs during undisturbed spontaneous respiration. Both changes in transpulmonary pressure and changes in smooth muscle tone induced by anesthetics affected the performance of lungs and bronchi.Dynamic lung compliance and bronchial distensibility increased significantly during halo-thane anesthesia without appreciable alteration of the transpulmonary pressure gradient, indicating that halothane reduces bronchomotor tone. Bronchi were less distensible and lung compliance fell during cyclopropane anesthesia suggesting an increase in bronchomotor tone. Changes in lung compliance and bronchial distensibility were minor during ether, methoxyflurane and trichlorethylene anesthesia.Alterations in lung compliance reverted spontaneously during emergence from all anesthetics. With the exception of ether, a positive correlation existed between lung compliance and bronchial distensibility during anesthesia. Deep ether and cyclopropane anesthesia were found to afford some protection against bronchoconstriction and the decrease in lung compliance following intravenous histamine.