Maximal supine exercise studies at the time of heart catheterisation were performed one to five years after open heart surgery for Fallot's tetralogy on 29 subjects 6 to 16 years of age. During exercise right ventricular systolic pressure exceeded 50 mmHg in all but 2 subjects, and end-diastolic pressure increased to over 15 mmHg in 10 subjects. Pulmonary artery peak systolic pressure was abnormal in 5 patients. Maximal exercise cardiac index was below the normal range in only 2 subjects, but below the mean for normals in 80 per cent of the patients. Only 3 patients had clinical exercise performances below the 3rd centile of normal subjects using a maximal upright bicycle exercise test, and only 1 subject was below the normal range for endurance time on the Bruce treadmill test. The patients in this series performed better than those in other series, possibly because of their younger age at operation, the use of a large control series of normal subjects taken from a clinic population, the willingness of the patients to work to near exhaustion, and previous encouragement of the patients to become normally active children.