To characterize the exercise hemodynamic responses after definitive operation for patients with univentricular atrioventricular connection, we compared the cardiorespiratory response to exercise of 4 patients who had undergone the ventricular septation procedure and 9 patients who had undergone the Fontan operation with that of 38 patients who had univentricular atrioventricular connection but had not undergone definitive operation. Exercise duration and total work increased significantly after the Fontan procedure but not after the septation operation. Rest or exercise (or both) ventilation improved significantly after the septation operation but less so after the Fontan operation. Blood oxygen saturation (rest and exercise) improved significantly after either procedure. Cardiac output and stroke-volume response to exercise remained abnormal in both groups of patients after operation. Although definitive operation for univentricular atrioventricular connection results in improved cardiorespiratory function at rest and exercise, these variables remain abnormal when compared with those of normal subjects.