The present study was conducted to understand better the mechanisms leading to the decrease in exercise capacity observed in horses suffering from chronic obstructive pulmonary disease (COPD). Five COPD horses were submitted to a standardized submaximal treadmill exercise test while they were in clinical remission or in acute crisis. Respiratory airflow, O₂ and CO₂ fractions in the respired gas, pleural pressure changes and heart rate were recorded, and arterial and mixed venous blood were analyzed for gas tensions, hemoglobin, and plasma lactate concentrations. O₂ consumption, CO₂ production, expired minute ventilation, tidal volume, alveolar ventilation, cardiac output, total pulmonary resistance, and mechanical work of breathing were calculated. The results showed that, when submaximally exercised, COPD horses in crisis were significantly more hypoxemic and hypercapnic and that their total pulmonary resistance and mechanical work of breathing were significantly higher and their expired minute ventilation significantly lower than when they were in remission. However, their O₂ consumption remained unchanged, which was probably due to the occurrence of compensatory mechanisms, i.e., higher heart rate, cardiac output, and hemoglobin concentration. Last, their net anaerobic metabolism seemed to be more important.