Studies were performed measuring parameters of respiratory and circulatory function at rest and during maximum tolerable constant work rate treadmill exercise in 16 clinically well patients who had undergone heart-lung transplantation for end stage pulmonary hypertension. Ten patients were studied before and within eight weeks following transplantation. Long-term function with exercise was further evaluated with follow up studies at one year (n = 10) and two years (n = 6), posttransplantation. Posttransplant gas exchange and ventilation during exercise are essentially normal with neither being limiting to maximal exercise. Exercise capacity is significantly improved posttransplant, primarily as a result of improvement in the circulation over that found pretransplant in uncorrected pulmonary hypertension. Although improved, circulatory limitations of maximal exercise may still persist. Cardiorespiratory function at maximum tolerable exercise is well maintained following heart-lung transplantation for at least two years, providing no complications occur. This suggests that denervation of the heart and lungs, disruption of the bronchial circulation and pulmonary lymphatics, and the graft ischemia encountered at the time of transplantation impose no serious limitations on long-term cardiopulmonary function. The overall functional capacities of the transplanted heart and lungs are more than adequate for meeting the activities of normal life.