1. The effect of breathing an anaesthetic aerosol of 5% bupivacaine hydrochloride has been assessed in dog and man. 2. In the dog, the cough reflex was abolished and the Hering-Breuer inflation reflex severely impaired or abolished; breathing became slower and deeper; no pathological changes were found in the lungs of these dogs. 3. In man, no untoward effects resulted from a 10 min period of aerosol inhalation; there were no systematic effects on airway resistance or lung volumes and the cough reflex in response to either tactile or chemical (citric acid aerosol) stimulation was invariably abolished. The Hering-Breuer inflation reflex was impaired, but this was not associated with any change in resting ventilation. The Ve/CO2 response was enhanced after aerosol anaesthesia; subjects felt an exaggerated dyspnoea. The aerosol anaesthesia abolished the afferent pathway of a reflexly elicited bronchoconstriction in one subject. There was no effect on the ability to hold the breath, or on the quality of the associated sensation. 4. Control aerosols of sodium chloride solution or phosphate buffer produced no effects. Control experiments with intravenous infusions of bupivacaine proved that none of the effects could have been produced by systemic effects of the absorbed anaesthetic. 5. Plasma concentrations of bupivacaine in man did not exceed a recognized toxic level. The experiments demonstrate a safe reversible anaesthesia of the airways in man lasting for a period of 10-20 min.