It has been suggested that patients with chronic supraventricular tachycardia may have impaired ventricular function, which returns to normal after surgical procedures that eliminate the tachycardia. The purpose of this study was to determine the functional consequences of prolonged supraventricular tachycardia in 12 awake dogs in which permanent asynchronous atrial pacemakers were implanted and programmed to a rate of 190 ± 5 beats/min. Serial radionuclide angiograms were obtained immediately after pacemaker activation and at regular intervals over a 3 month period. Chronic tachycardia resulted in a significantly depressed ejection fraction (49% ± 1% to 29% ± 3%; p < 0.0005) compensated for by a dramatic increase in left ventricular end-diastolic volume (69 ± 4 to 105 ± 9 ml, p < 0.005). Stroke volume and cardiac output were not significantly changed. Five dogs were allowed to recover, and serial radionuclide angiograms were obtained for 12 weeks. Although ejection fraction returned to control values (50% versus 47%, p = no significant difference), end-diastolic volume remained persistently elevated after a 12 week recovery period in all animals (67 ± 5 versus 91 ± 6 ml, p < 0.05). Thus prolonged tachycardia resulted in significant functional changes associated with cardiac enlargement, which were not immediately reversible.