After the Mustard operation, patients have reduced exercise tolerance, abnormal right and left ventricular responses to exercise and cardiac rhythm disturbances. The cardiovascular response to exercise was measured noninvasively in 19 patients from 4.5 to 20 years (mean 10.3) after operation. Mean work performed and maximal oxygen uptake for the group were substantially subnormal (42 +/- 23% and 59 +/- 18% [mean +/- 1 standard deviation] of the predicted values, respectively). Resting heart rate, blood pressure, systemic arterial blood oxygen saturation, cardiac index, stroke volume and systemic vascular resistance were not significantly different from control values. At maximal exercise, heart rate, systemic arterial blood oxygen saturation, cardiac index and stroke volume were significantly reduced in comparison with control values. After the Mustard operation, cardiovascular status at rest may be relatively normal, but during maximal exercise, marked abnormalities occur in nearly all indexes of cardiovascular function. Decreased cardiac output response to exercise is a result of decreased stroke volume response and, to a lesser extent, diminished heart rate. It is associated with abnormally increased total systemic vascular resistance.