To investigate the effect of increased cardiac output on lung water and solute movement in abnormally permeable lungs, experiments were performed in sheep with chronic vascular catheters and lung lymph fistulas. Pulmonary permeability was increased by infusing air microemboli intravenously for 3 h. Once stable lymph flow was observed after stopping the air embolization sheep were either exercised on a treadmill to increase cardiac output (n = 10) or merely continuously monitored (control experiments, n = 6). In control experiments, lung lymph flow remained at markedly elevated but stable values. Sheep exercised for a median time of 45 min. Cardiac output increased 85%, whereas lung lymph flow, in terms of baseline flow rates, increased 296%. The lymph to plasma concentration ratio for total protein was unchanged, and left atrial pressure remained stable and normal during exercise. During exercise, pulmonary arterial pressure increased by 3 mm Hg. The data, when compared with previous observations in normal sheep, demonstrate that exercise-induced increases in cardiac output result in disproportionate increases in lung lymph flow when pulmonary vascular permeability is increased.