The right hemithorax of rats was exposed to 3 kR of X rays; the animals were sacrificed at different times after exposure up to 9 mo., and various biochemical parameters were determined. After a slight early decrease, collagen increased during the fibrotic stage. An increase during fibrosis was also seen for DNA, .beta.-glucuronidase, cathepsin, histamine, serotonin and lipid peroxides. Fibrinolytic activity was depressed at most time points studied.