The use of various types of rate-responsive pacing systems (AAI-R, VVI-R) in anatomically normal hearts has resulted in improved exercise performance. This has been measured as improved work rate, maximal oxygen consumption, and oxygen consumption at which ventilatory anaerobic threshold is achieved. The usefulness of rate-responsive pacing in patients with congenital heart defects has not been extensively studied.1-3 Most patients with congenital heart defects are postoperative patients with either surgical atrioventricular block or sinus node dysfunction.4,5 The underlying structural cardiac defects are often complex and may contribute to impaired ventricular function. How these abnormalities affect physiologic cardiopulmonary response with single chamber rate-responsive pacing is unknown. We compared the exercise responses of patients with congenital heart defects using an implanted motion-sensing rate-responsive single chamber pacing system. This study was performed to evaluate potential efficacy of variable over fixed rate single chamber pacing in these patients.