BACKGROUND--Adult patients with total correction of tetralogy of Fallot may have poor exercise capacity associated with impaired right heart function and in particular pulmonary regurgitation. The ventilatory responses to exercise were studied in a group of such patients to assess relations between ventilation, exercise capacity, and right ventricular function. METHODS--30 patients (7 female) (aged 27.8 (6.0) years) and 30 (7 female) controls of a similar age range were studied prospectively. All underwent exercise testing with metabolic gas exchange to determine peak oxygen consumption (peak VO2), and (as indices of the ventilatory response) the slope of the relation between both respiratory rate (RR) and ventilation (VE) against carbon dioxide production (VCO2). Patients were studied with pulsed wave Doppler echocardiography to determine pulmonary arterial systolic and diastolic flow characteristics. Patients were defined as having restrictive right ventricular function where diastolic pulmonary forward flow was seen coincident with atrial systole. RESULTS--In the group with tetralogy of Fallot mean (SD) peak VO2 was 35.3 (7.5) ml/kg/min (93.6 (15.3) % of expected for age, weight, height and sex). The RR/VCO2 slope was steeper in the Fallot group (6.8 (2.6) v 9.6 (4.7), P < 0.02). Those with restrictive right ventricles achieved a higher peak VO2 than those without (82.5 (10.1) % v 100.9 (13.8), P < 0.001). In the Fallot group alone, there was an inverse relation between ventilatory response and peak VO2 (RR/VCO2 v peak VO2; r = -0.63, P = 0.003: VE/VCO2 v peak VO2; r = -0.62, P < 0.001). CONCLUSIONS--Many of these patients with repaired tetralogy of Fallot had near normal exercise capacity, but as exercise capacity decreased, the ventilatory response to exercise increased. This was not due to alterations in pulmonary function tests or to the effects of cardiac size causing decreased lung volume. It may be that the increased ventilatory rate at a given level of carbon dioxide production acts as a respiratory pump aiding right ventricular function.