Late survival after Mustard repair of transposition of the great arteries is generally good but is often characterized by progressive deterioration of ventricular function and by late postsurgical arrhythmias, thus imposing the need for permanent pacing. To evaluate how chronic pacing affects long-term exercise capacity, we compared two groups of these patients: group 1, comprising 12 patients, aged 9.0 +/- 2.6 years, without pacemaker; and group 2, comprising 18 patients, aged 9.3 +/- 2.0 years, with pacemaker. Patient evaluation included history, physical examination electrocardiograph, Holter monitoring, and echocardiography. Pacing modes were as follows: AAI (6 patients), AAIR (9 patients), VVI (2 patients), and VVIR (1 patient). At exercise test we evaluated exercise tolerance, maximum heart rate, blood pressure, oxygen consumption, and cardiac output at rest and at peak exercise. The two groups were comparable for all variables examinated. All pacemakers showed normal function. During the exercise, 11 of 12 patients in group 1 showed sinus rhythm, and in group 2, 11 patients showed sinus rhythm, 5 junctional rhythm, and 2 continuous pacing. There were no significant differences between groups. Chronically paced Mustard patients but with restoration of spontaneous rhythm during the exercise test do not show reduced exercise tolerance in comparison with nonpaced Mustard patients.