Progressive, irreversible fibrosis is one of the most clinically significant consequences of ionizing radiation on normal tissue. When applied to lungs, it leads to a complication described as idiopathic pneumonia syndrome (IPS) and eventually to organ fibrosis. For its high mortality, the condition precludes treatment with high doses of radiation. There is widespread interest to understand the pathogenetic mechanisms of IPS and to find drugs effective in the prevention of its development. This report summarizes our experience with the protective effects of L 158,809, an angiotensin II (ANG II) receptor blocker, and two angiotensin converting enzyme (ACE) inhibitors in the development of IPS and the role of transforming growth factor β (TGF-β) and of alpha-actomyosin (α SMA) in pathogenesis of radiation induced pulmonary fibrosis in an experimental model of bone marrow transplant (BMT). Male WAG/Riji/MCV rats received total body irradiation and a regimen of cyclophosphamide (CTX) in preparation for bone marrow transplant. While one group of animals remained untreated, the remainders were subdivided into three groups, each of them receiving either the ANG II receptor blocker or one of the two ACE inhibitors (Captopril or Enalapril). Each of the three drugs was administered orally from 11 days before the transplant up to 56 days post transplant. At sacrifice time the irradiated rats receiving only CTX showed a chronic pneumonitis with septal fibrosis and vasculitis affecting, in particular, small caliber pulmonary arteries and arterioles. Their lung content of hydroxyproline was also markedly elevated in association with the lung concentrations of thromboxane (TXA2) and prostaglandin (PGI2), (two markers of pulmonary endothelial damage). A significant increase of alpha actomyosin staining was observed in vessels, septa and macrophages of the same animals which also overexpressed TGF-β. When L 158,809, Captopril and Enalapril were added to the radiation and cytoxan treatment, a significant amelioration of the histological damage as well as the overexpression of α SMA was observed. Lung concentrations of hydroxyproline, PGI2, TXA2 and TGF-β were also observed in these animals so that the values of these compounds were closer to those measured in untreated control rats than to their irradiated and cytoxan treated counterparts. Angiotensin II plays an important role in the regulation of TGF-β and α SMA, two proteins involved in the pathogenesis of pulmonary fibrosis. The finding that ACE inhibitors or ANG II receptor blockers protect the lungs from radiation induced pneumonitis and fibrosis reaffirms the role that ANG II plays in this inflammatory process and suggests an additional indication of treatment of this condition, thus opening a new potential pharmacologic use of these drugs.