Pulmonary fibrosis is characterized by chronic scar formation and deposition of extracellular matrix, resulting in impaired lung function and respiratory failure. Idiopathic pulmonary fibrosis (IPF) is associated with pronounced morbidity and mortality and responds poorly to known therapeutic interventions; there are no known drugs that effectively block or reverse progressive fibrosis. Transforming growth factor β (TGF-β) is known to mediate extracellular matrix gene regulation and appears to be a major player in both the initiation and progression of IPF. TGF-β mediates its biological effects through members of a family of activin receptor–like kinases (ALK). We have used a gene transfer model of progressive TGF-β1–induced pulmonary fibrosis in rats to study a newly described orally active small molecular weight drug that is a potent and selective inhibitor of the kinase activity of ALK5, the specific TGF-β receptor. We show that the drug inhibits the induction of fibrosis when administered at the time of initiation of fibrogenesis and, most important, blocks progressive fibrosis when administered transiently to animals with established fibrosis. These data show promise of the development of an effective therapeutic intervention for IPF and that inhibition of chronic progressive fibrosis may be achieved by blocking TGF-β receptor activation.