The right hemithorax of rats was exposed to 1 kR of roentgen rays, the animals were killed at different times, and various physiologic and biochemic parameters (DNA, protein, collagen, sialic acid, lysosomal enzymes, fibrinolytic activity, peroxides and blood flow ratio between irradiated and non irradiated lung) were determined from 1 day until 11 months after irradiation. A decrease in relative blood flow was observed from 2 weeks until 11 months, a decrease in phospholipids from one to 3 months and a decrease in fibrinolytic activity from 1 day to 11 months after irradiation. At several times a decrease in sialic acid and cathepsin D was also noted. The changes are discussed with respect to the pathogenetic mechanisms of late lung damage.