In 6 healthy, young males the adaptation to arm, leg, and combined arm and leg exercise was studied by cardiac catheterization in supine and sitting position. The hemodynamic and ventilatory responses were equal during leg exercise and when more muscle groups participated as during combined arm and leg exercise. During exercise with the arms, however, total ventilation, heart rate, and lactate formation were significantly higher for a given O2 uptake. With arm exercise the systolic, diastolic, and mean pressures in the aorta increased more in relation to the cardiac output than when the legs participated in the work. The observed differences in circulatory adaptation during arm versus leg exercise indicate higher sympathetic tone during arm exercise. The effect of body position was more pronounced during arm exercise only than when the legs took part in the work. In the sitting position the stroke volume did not increase on transition from rest to arm exercise when the legs were passive.