After a Fontan repair for congenital heart disease, 42 patients underwent graded supine bicycle exercise tests at levels relevant to normal daily activities. Results were compared with those of 28 age-matched normal control subjects. At rest, the cardiac index, stroke index and systolic blood pressure were comparable in both groups, but increases with exercise were smaller in the patients with a Fontan circulation. The heart rate at rest was higher in the Fontan group, but this difference disappeared as soon as exercise started. To determine whether there are limitations intrinsic to the Fontan circulation at these levels of exercise, the 10 best performers were compared with 10 age-matched control subjects; no differences were found in cardiac index, stroke index, heart rate or blood pressure at any exercise level. Analysis of the determinants of cardiac output showed that at the other end of the spectrum poor performance after a Fontan operation did not result from inadequate levels of heart rate, but from an inability to increase or maintain stroke volume. Multivariate analysis demonstrated that impairment of ventricular contractility, only when severe, predicted limited performance. There was no evidence of increased afterload, particularly in the poor performers. Therefore, ventricular filling, which is determined primarily by the pulmonary vascular bed, appears to be a major determinant of functional result after a Fontan repair.