We measured lung function and airway reactivity in response to methacholine and citric acid administered by aerosol in 2 groups of ponies (principal and control). Principal ponies had a history of heaves, a disease characterized by recurrent airway obstruction. Control ponies had no history of respiratory disease. Both principal and control ponies were paired (principal and control), and measurements were made when principal ponies were in clinical remission (Period A), following barn exposure when principal ponies had acute airway obstruction (Period B), and 1 and 2 wk after they were returned to pasture (Periods C and D). Differences between groups were primarily found at Period B. Barn housing (Period B) decreased dynamic compliance (Cdyn) and increased pulmonary resistance (RL) of principal but not of control ponies. When compared with control ponies at Period B, principal ponies demonstrated airway hyperreactivity. The dose of methacholine required to reduce Cdyn to 65% of baseline (ED65Cdyn) was lower, the change in RL induced by an aerosol of 0.1 mg/ml methacholine (delta RL 0.1) was higher, and the percent change in Cdyn in response to an aerosol of 0.1 mg/ml methacholine (delta %Cdyn 0.1) was larger in principal than in control ponies. A 10-min inhalation of 10% citric acid aerosol did not cause changes in Cdyn in either group of ponies. Control ponies did not increase RL in response to citric acid, whereas at Period B, RL of principal ponies increased following citric acid. We conclude that ponies in clinical remission from heaves are not hyperreactive to aerosols of methacholine or citric acid. Hyperreactivity only exists during acute exacerbations of airway obstruction.