 Background. In congenital long QT syndrome type 1 LQT1, episodes of ventricular tachycardia are usually triggered by exercise and can be prevented in most patients by beta-blocker therapy. In addition, LQT1 associated with a normal resting QT interval can be unmasked by the abnormal QT response to exercise testing failure of the QT interval to shorten normally. Preliminary data from our laboratory show that the exercise QT intervals of patients with LQT1 are partially normalized by beta-blocker therapy. It is still currently not known if beta-blockers modify the QTheart rate relationship a primary effect on repolarization or if the normalizing effect is due to the inability of subjects on beta-blockers to attain sufficiently high workloads due to reduced heart rate for prolongation to occur. Moreover, the physiologic response of the exercise QT interval to beta-blockers in healthy control subjects is not known. 