Taurine has the ability to resist the action of oxygen-free radicals by protecting cytomembranes. L-arginine increases the nitric oxide in the organism, inhibiting some types of abnormal cell proliferation. We examined the actions and protective mechanisms of these two substances against radiation pulmonary fibrosis. The mRNA transcription and the distribution of pulmonary types I and III procollagen were measured by in situ hybridization. Type I and type III collagen in the interalveolar septa and the change ratio between the two types were assayed using Sirius Red staining and polarizing microscopy. Angiotensin converting enzyme (ACE), hydroxyproline (Hyp), malondialdehyde (MDA), and nitric oxide (NO) were measured in lung tissue after irradiation. We found that, following administration of taurine or arginine, the mRNA transcription of procollagen types I and III significantly decreased in the pulmonary tissue 3 months after irradiation and the Hyp content was lower, especially after the administration of arginine. The extent of the radiation-induced decrease in ACE activity was markedly attenuated. As a substrate of NO production, arginine can significantly increase the pulmonary NO content. We conclude that the administration of taurine or arginine can significantly decrease the mRNA transcription of procollagen and the synthesis of type I collagen in the interalveolar septa and can decrease the Hyp content of the pulmonary tissue. Following irradiation, NO can attenuate the radiation-induced decrease in ACE activity. Our study suggests that exogenous NO and taurine significantly protect against radiation pulmonary injury.