Inhalation of aerosols of citric acid, histamine phosphate, or carbon dust, or air cooled to - 20°C or rapid respiratory maneuvers (inspiration or expiration) results in an increase in airway resistance in some patients with asthma or bronchitis. It has been shown previously in animals that stimulation of cough receptors results in bronchoconstriction through efferent cholinergic pathways. In the patients studied, the administration of atropine sulfate, which would block such pathways, abolished the bronchoconstrictor effects of all the stimuli except large doses of histamine, which may exert a direct effect on airway smooth muscle. These data suggest that sensitized cough receptors may be involved in triggering reflex airway constriction in such patients.