1. Sulphated glycoprotein output from the trachea, isolated in situ, has been measured in anaesthetized cats by a radio-isotopic method. The effects of irritation of various parts of the airway on this mucus output were studied. 2. Mechanical stimulation of the nose and nasopharynx increased tracheal mucus output by reflexes which involved parasympathetic and probably also sympathetic motor pathways. 3. Laryngeal stimulation had a similar through the same motor pathways. 4. Inhalation of ammonia vapour into the lower airways reflexly increased mucus output from the isolated trachea. The efferent pathway for this reflex was mainly or entirely parasympathetic. It is argued that the afferent pathway involved cough receptors. 5. Lung inflation, inhalation of histamine aerosol and intravenous injection of phenyl diguanide (which excite mainly lung stretch receptors, lung 'irritant' receptors and alveolar 'J-receptors' respectively) had no consistent effect on tracheal mucus secretion. 6. The afferent and efferent pathways of these reflexes are discussed.