To determine whether cough and bronchoconstriction result from alterations in the osmolarity or alterations in the ion concentration of inhaled aerosols and to determine if the specific ions in the aerosol are important, we had 9 subjects with mild asthma inhale various solutions while we recorded cough and measured specific airway resistance. To evaluate the effects of altering osmolarity and ion concentration separately, we administered aerosols of hypo-osmolar distilled water (0 mosm), iso-osmolar sodium chloride (308 mosm), iso-osmolar dextrose in water (308 mosm), hyperosmolar sodium chloride (1,232 mosm), and a hyperosmolar solution of dextrose and sodium chloride (1,232 mosm). To evaluate cough without bronchoconstriction, we had the subjects inhale metaproterenol before inhaling the same aerosols. To determine whether the absence of a specific ion was important in causing cough or bronchoconstriction, we had the subjects inhale iso-osmolar solutions of sodium bromide, sodium gluconate, and lysine monohydrochloride. We found that alteration in osmolarity away from iso-osmolarity of inhaled aerosols is a stimulus for bronchoconstriction in subjects with mild asthma. Absence of ions in the presence of iso-osmolarity is not a stimulus for bronchoconstriction, but the absence of a permeant anion is a stimulus for cough. Thus, we found that the responses of cough and bronchoconstriction to inhaled aerosols can be separated.