This study was designed to define the effects of halothane on the compliance of the trachea. An isolated in-situ tracheal preparation was studied in 14 mongrel dogs anesthetized with pentobarbital. Compliance of the closed tracheal segment was measured with continuous intraluminal pressure recordings during repeated injections of known volumes of air. Slow-adapting neural activity observed in paratracheal branches of the recurrent laryngeal nerve accurately reflected pressure in the tracheal segment. Halothane at 0.5–4.0 per cent concentrations caused a significant (P < 0.001) average 10 per cent increase in the compliance of the trachea. Stimulation of the efferent vagus caused a significant (P < 0.001) average 8 per cent decrease in compliance of the trachea. After exposure to halothane, vagal stimulation still caused a significant decrease in compliance of the trachea.