To investigate the acute physiological and structural changes after lung irradiation, the effects of whole-lung irradiation were investigated in fourteen sheep. Ten sheep were prepared with vascular and chronic lung lymph catheters, then a week later were given 1,500 rad whole-lung radiation and monitored for 2 days. Four sheep were given the same dose of radiation and were killed 4 h later for structural studies. Lung lymph flow increased at 3 h after radiation (14.6 +/- 2.1 ml/h) to twice the base-line flow rate (7.5 +/- 1.3), with a high lymph-to-plasma protein concentration. Pulmonary arterial pressure increased twofold from base line (18 +/- 1.6 cmH2O) at 2 h after radiation (33 +/- 3.8). Cardiac output and systemic pressure in the aorta did not change after lung radiation. Arterial O2 tension decreased from 85 +/- 3 to 59 +/- 4 Torr at 1 day after radiation. Lymphocyte counts in both blood and lung lymph decreased to a nadir by 4 h and remained low. Thromboxane B2 concentration in lung lymph increased from base line (0.07 +/- 0.03 ng/ml) to peak at 3 h after radiation (8.2 +/- 3.7 ng/ml). The structural studies showed numerous damaged lymphocytes in the peripheral lung and bronchial associated lymphoid tissue. Quantitative analysis of the number of granulocytes in peripheral lung showed no significant change (base line 6.2 +/- 0.8 granulocytes/100 alveoli, 4 h = 10.3 +/- 2.3). The most striking change involved lung airways. The epithelial lining of the majority of airways from intrapulmonary bronchus to respiratory bronchiolus revealed damage with the appearance of intracellular and intercellular cell fragments and granules. This new large animal model of acute radiation lung injury can be used to monitor physiological, biochemical, and morphological changes after lung radiation. It is relevant to the investigation of diffuse oxidant lung injury as well as to radiobiology per se.