OBJECTIVE To determine the relation between functional capacity and heart rate response to exercise in patients with atrial fibrillation. METHODS 73 consecutive patients with chronic atrial fibrillation. METHODS Relation between functional capacity, measured as peak oxygen consumption (peak VO2; ml/min/kg), and heart rate at all stages of exercise in univariate and multivariate analyses. RESULTS Peak VO2 showed no correlation with resting heart rate, but it showed a negative correlation with heart rate during the first stage of exercise (r = -0.94, p < 0.01). Indeed, heart rate during the early stages of exercise (stages 1-5) was higher in patients with a peak VO2 less than or equal to 20 ml/mm/kg than in those with a peak VO2 greater than 20 ml/min/kg (heart rate 140 v 125 beats/min, p < 0.05). At maximal exercise, however, peak VO2 was positively correlated with heart rate (r = 2.15, p < 0.0001). CONCLUSIONS In patients with atrial fibrillation and impaired functional capacity heart rate at low levels of exercise is augmented but maximal heart rate attenuated compared with patients with preserved functional capacity. Excessive heart rate responses to minor exercise may have deleterious effects on left ventricular function and thereby further limit functional capacity.