The study was carried out on 8 boxer dogs in three groups. They were anesthetized, intubated and breathed spontaneously. The animals were exposed to inhalative acetylcholine (ACH) aerosol, mechanical stimulus to the bronchial tree, intrabronchial and intravenous application of ACH solution. Dynamic elasticity, lung resistance, tracheal resistance and breathing rates were determined. A special method was used for recording tracheal activity in spontaneously breathing animals and noted as tracheal pressure. Those parameters were noted before and after partial blocking of the cervical vagus nerves on both sides. Bilateral vagal block entirely prevented the response produced by mechanical irritation and intrabronchial ACH. Inhalative and intravenous ACH produced significantly less response after vagal block, but certain residues remained. It is concluded that mechanical irritation works entirely through the reflex path, but ACH mostly follows the reflex path a little on the smooth muscle directly.