The inhalation of capsaicin for 1 min, delivered as an aerosol by nebulising solutions of capsaicin at concentrations of 2-65 mumol 1(-1), caused dose-dependent coughing in normal volunteers and subjects with mild asthma. Capsaicin did not cause a feeling of breathlessness, and had no effect on forced expiratory volume in 1 s (FEV1) measured at the 1st, 5th and 9th min after the challenge was completed. Coughing started within seconds of applying the face mask, continued throughout the minute of capsaicin inhalation, and stopped within seconds of the mask being removed. In any one subject the number of coughs was reproducible when repeated on the same day or after an interval of several days. Experiments using local anaesthesia applied to the buccal mucosa or larynx indicated that the cough was caused by the stimulation of capsaicin-sensitive nerve terminals situated in the larynx. Cough response was not altered by the prior inhalation of sodium cromoglycate.