1. Dead space (CO(2)-front method), lung compliance, cervical tracheal volume change and total lung resistance were measured simultaneously in anaesthetized dogs, paralysed or open-chest, and the responses to nerve section and stimulation and to ventilation of the dogs with different gas mixtures were studied.2. Bilateral cervical vagotomy changed all four parameters in a way consistent with airway dilation, and bilateral vagal stimulation had the opposite actions. A weak airway dilator role was indicated for the sympathetic nerves.3. Ventilation with 4% CO(2), 8% CO(2) or 10% O(2) changed all four parameters as if the airways had constricted, and 100% O(2) caused oppossite, but weaker, responses.4. The responses to the three airway constricting gas mixtures were present, but reduced in intensity, after bilateral cervical vagotomy and some even weaker responses were present after additional bilateral sympathectomy.5. During restoration of airway smooth muscle tone by electrical stimulation of the distal ends of the cut vagus nerves, administration of CO(2)-rich or hypoxic gas mixtures had no greater effect than during controls with vagi cut but without stimulation.6. No dilator responses to hypercapnia or hypoxia were seen, either in innervated or denervated trachea and airways.