Irreversible airflow limitation in asthma is associated with airway remodeling, in which the differentiation of fibroblasts to myofibroblasts plays a pivotal role. In asthmatic airways, excessive production of reactive nitrogen species (RNS) has been observed. The aim of this study is to evaluate whether peroxynitrite, one of the RNS, can affect the differentiation of fibroblasts to myofibroblasts. Human fetal lung fibroblasts were treated with various concentrations of authentic peroxynitrite or a peroxynitrite donor 3-morpholinosydnonimine hydrochloride (SIN-1) and the expressions of -smooth muscle actin (-SMA) and desmin, markers of myofibroblast differentiation, were evaluated. The releases of transforming growth factor-1 (TGF-1) and extracellular matrix (ECM) proteins including fibronectin and collagen I were assessed. To clarify the mechanism in this differentiation, the effect of anti-TGF- antibody or NF-B inhibitors on the -SMA expression and ECM production was assessed. Peroxynitrite and SIN-1 significantly augmented the -SMA expression compared with control in a concentration-dependent manner (p < 0.01 and p < 0.05, respectively). Peroxynitrite significantly increased desmin and TGF-1 production (p < 0.01). Peroxynitrite enhanced the translocation of nuclear factor-B (NF-B) into the nucleus confirmed by immunocytostaining and immunoblotting. Peroxynitrite-augmented -SMA expression was blocked by NF-kB inhibitors, MG132 and caffeic acid phenethyl ester (CAPE), and anti-TGF- antibody. CAPE completely inhibited the peroxynitrite-augmented TGF-1 release. The production of fibronectin and collagen I was significantly increased by peroxynitrite (p < 0.01) and inhibited by anti-TGF- antibody. These results suggest that RNS can affect the differentiation to myofibroblasts and excessive ECM production via an NF-B-TGF-1 dependent pathway.