Using a 133Xe wash-out technique changes in the gas exchange capacity of the pig lung have been studied after irradiation with a range of single doses of 60Co gamma-rays. Lung function was assessed for periods up to 104 weeks after irradiation. It was found that the gas exchange capacity of the lung was impaired as early as 4 weeks after irradiation. After a dose of 9 Gy the initial impairment in lung function was resolved within 13 weeks, while after 14.7 Gy damage persisted. The degree of impairment in lung function during the relatively early pneumonitic and late fibrotic phases was indistinguishable and suggested that the late functional impairment was a continuation of the earlier damage. The results of the lung function tests were converted into quantal data and ED50 values of 9.68 +/- 0.3 Gy for early (4-26 weeks) and 9.73 +/- 0.34 Gy for late (39-104 weeks) damage were obtained. The ED50 value for fibrosis and focal scarring based on the histological assessment, 104 weeks after irradiation, was 11.12 +/- 0.9 Gy. The differences in ED50 values were not significant.