The aim of this study was to evaluate the respiratory function of horses affected with chronic obstructive pulmonary disease (COPD) when maintained in a barn on wood shavings and fed grass silage for a period of 6 weeks. The mechanics of breathing, blood gas analysis and bronchial reactivity were examined on five horses with COPD at the end of the environment-controlled period (Period B) and the results compared with values obtained after 2 months at pasture (Period A) and after the onset of clinical signs of acute crisis (Period C). The results showed that clinical and functional parameters were similar in healthy horses and in COPD horses placed on pasture or stabled and fed grass silage. Moreover, the bronchial reactivity of COPD horses at pasture was similar to the bronchial reactivity of healthy horses. COPD horses fed grass silage however presented a bronchial reactivity intermediate between those measured after Period A and Period C which suggested they would be more susceptible to develop bronchoconstriction when in contact with inhaled irritants.