In anesthetized sheep, ventilated by positive pressure, pulmonary arterial and left atrial pressures, cardiac output, and lung lymph flow (Qlym) were measured and lymph-to-plasma protein concentration ratio (L/P) was determined. For sheep were irradiated with 60Co on the bilateral lower lobes, avoiding irradiation on the heart and the caudal mediastinal lymph node. Qlym increased during and after irradiation, and reached a new steady state (160% increase), while pulmonary arterial or left atrial pressure did not change. In 4 control experiments, pulmonary hemodynamics, Qlym or L/P did not change. Therefore the increased fluid or protein flux after irradiation cannot be ascribed only to abnormal pulmonary hemodynamics. Endothelial damages seem to play a role in the increase of fluid filtration and protein permeability.