Ten female and eight male adults with tetralogy of Fallot, the majority totally corrected at adult age, have been studied at rest and during submaximal and maximal exercise on a bicycle ergometer. Oxygen uptake was determined by the Douglas bag technique and cardiac output by the dye-dilution method. Maximal oxygen uptake was reduced about 30-40% from normal. Thus a complete normalization of the aerobic working capacity was not achieved in spite of an intracardiac repair that was considered surgically satisfactory. Cardiac output response to exercise was subnormal, mainly due to small stroke volumes and partly because of low heart rates. A fall in stroke volume of more than 10 ml was found in 8 of the patients during exercise. No correlation was found between stroke volume during maximal excercise, on the one hand, and the presence of a particular residual defect, anatomy of the right ventricular outflow tract prior to operation and the use of a right ventricular outflow patch on the other. However, too few patients were studied to allow any definite conclusions as to the possible influence of these variables. It remains to be shown whether the haemodynamic abnormalities will be less and the aerobic work capacity better if total correction is undertaken at an early age.