Cardiopulmonary function studies at rest and during moderate and submaximal exercise were performed on 8 sea level athletes at sea level and on the 1st and 5th day of residence at 7,800 ft of altitude. Following ascent, arterial oxygen saturation diminished 4% at rest and 4 to 7% on exertion, whereas oxygen uptake decreased 10% at rest and 7 to 9% during exercise. Cardiac index was reduced 10% solely at rest on the 1st day at altitude and 20 and 15% at rest and exercise, respectively, on the 5th day. Mean pulmonary artery pressure at altitude increased 18% at rest and 20 and 30% on exertion above its counterparts at sea level, because of increased pulmonary resistances due to hypoxic vasocontriction at the precapillary level. In addition, a subjective evaluation of exercise tolerance suggested that submaximal exercise at sea level became nearly maximal at 7,800 ft. The physiological findings support the impeding effect of this altitude on work capacity.