Several lines of evidence indicate that depletion of glutathione (GSH), a critical thiol antioxidant, is associated with the pathogenesis of Idiopathic Pulmonary Fibrosis (IPF). However, GSH synthesis depends on the amino acid cysteine (Cys) and relatively little is known about the regulation of Cys in fibrosis. Cys and its disulfide, cystine (CySS) constitute the most abundant low-molecular weight thiol/disulfide redox couple in the plasma, and the Cys/CySS redox state (E_h Cys/CySS) is oxidized in association with age and smoking, known risk factors for IPF. Furthermore, oxidized E_h Cys/CySS in the culture media of lung fibroblasts stimulates proliferation and expression of transitional matrix components. The present study was undertaken to determine whether bleomycin-induced lung fibrosis is associated with a decrease in Cys and/or an oxidation of the Cys/CySS redox state, and to determine whether these changes were associated with changes in E_h GSH/glutathione disulfide (GSSG). We observed distinct effects on plasma GSH and Cys redox systems during the progression of bleomycin-induced lung injury. Plasma E_h GSH/GSSG was selectively oxidized during the pro-inflammatory phase, while oxidation of E_h Cys/CySS occurred at the fibrotic phase. In the epithelial lining fluid, oxidation of E_h Cys/CySS was due to decreased food intake. Thus, the data show that decreased precursor availability and enhanced oxidation of Cys each contribute to the oxidation of extracellular Cys/CySS redox state in bleomycin-induced lung fibrosis.