Oxidant stress has been implicated in the pathogenesis of chronic lung disorders like Idiopathic Pulmonary Fibrosis. However, mechanisms that link oxidant stress to fibrogenesis remain partially elucidated. Emerging data suggest an important role for the extracellular thiol/disulfide redox environment. The cysteine (Cys)/cystine (CySS) redox couple represents the predominant low molecular weight thiol/disulfide pool found in plasma, and is sensitive to aging, smoking, and other host factors. We hypothesized that an oxidized extracellular Cys/CySS redox potential (E_h Cys/CySS) affects lung fibroblasts by inducing intracellular signals that stimulate proliferation and matrix expression. We tested this hypothesis in primary murine lung fibroblasts and found that an oxidized E_h Cys/CySS (-46 mV) stimulated lung fibroblast proliferation. Furthermore, it stimulated their expression of fibronectin, a matrix glycoprotein highly expressed in fibrotic lung diseases and implicated in lung injury. This stimulatory effect was dependent on protein kinase C activation. Oxidant stress also increased the phosphorylation of CREB, a transcription factor known for its ability to stimulate fibronectin expression, and increased the expression of mRNAs and proteins coding for the transcription factors NF-B and Smad3. Fibroblasts cultured in normal (-80 mV) or reduced (-131 mV) E_h Cys/CySS showed less induction. Further, fibronectin expression in response to an oxidized E_h Cys/CySS was associated with expression of transforming growth factor 1 (TGF-1) and was inhibited by an anti-TGF-1 antibody and SB431542, a TGF-1 receptor inhibitor. These studies suggest that extracellular oxidant stress activates redox-sensitive pathways that stimulate lung fibroblast proliferation and matrix expression through upregulation of TGF-1.