BACKGROUND Continuous increases in the ventilatory equivalent for carbon dioxide (the ratio of minute ventilation to carbon dioxide production, an index of ventilatory drive) during exercise in patients with congestive heart failure would suggest that factors other than carbon dioxide excessively stimulate ventilation during exercise, and may be an important factor in exercise-related dyspnea and fatigue in these patients. RESULTS Eighty-five patients with congestive heart failure and 17 normal control subjects underwent symptom-limited exercise testing with gas-exchange analysis. Patients were divided into four functional classes (A-D, Weber's classification) based on peak exercise oxygen consumption. In all heart failure patient groups and in control subjects the ventilatory equivalent for carbon dioxide decreased (P < .005, class D; P < .0001, all other groups) from rest to anaerobic threshold. Three isolated patients showed a continuous increase in ventilatory drive during exercise (mean peak oxygen consumption 13.7 mL/kg/min). In the lowest functional class (D) the ventilatory equivalent for carbon dioxide was greater than in all other groups at rest, at anaerobic threshold, and at peak exercise (P < .01). CONCLUSIONS In all heart failure groups and in normal control subjects ventilatory drive, as determined by the ventilatory equivalent for carbon dioxide, decreases during exercise, Continuous increases in ventilatory drive during exercise are infrequently seen, suggesting that factors other than carbon dioxide production do not excessively stimulate ventilation in heart failure patients during exercise.