Transforming growth factor (TGF)-β upregulates plasminogen activator inhibitor type 1 (PAI-1) in a variety of cell types, and PAI-1 is considered to be an essential factor for the development of fibrosis. Our previous studies demonstrated that TGF-β decreased intracellular glutathione (GSH) content in murine embryonic fibroblasts (NIH/3T3 cells), whereas treatment of the cells with GSH, which restored intracellular GSH concentration, inhibited TGF-β-induced collagen accumulation by blocking PAI-1 expression and enhancing collagen degradation. In the present study, we demonstrate that GSH blocks TGF-β-induced PAI-1 promoter activity in NIH/3T3 cells, which is associated with an inhibition of TGF-β-induced JNK and p38 phosphorylation. Interestingly, although exogenous GSH does not affect phosphorylation and/or nuclear translocation of Smad2/3 and Smad4, it completely eliminates TGF-β-induced binding of transcription factors to not only AP-1 and SP-1 but also Smad cis elements in the PAI-1 promoter. Decoy oligonucleotides (ODN) studies further demonstrate that AP-1, SP-1, and Smad ODNs abrogate the inhibitory effect of GSH on TGF-β-induced PAI-1 promoter activity and inhibit TGF-β-induced expression of endogenous PAI-1. Furthermore, we show that GSH reduces TGF-β-stimulated reactive oxygen species (ROS) signal. Blocking ROS production with diphenyleneiodonium or scavenging ROS with a superoxide dismutase and catalase mimetic MnTBaP dramatically reduces TGF-β-induced p38 and JNK phosphorylation as well as PAI-1 gene expression. In composite, these findings suggest that GSH inhibits TGF-β-stimulated PAI-1 expression in fibroblasts by blocking the JNK/p38 pathway, probably by reducing ROS, which leads to an inhibition of the binding of transcription factors to the AP-1, SP-1, and Smad cis elements in the PAI-1 promoter. Copyright © 2007 the American Physiological Society.