Increased expression of transforming growth factor (TGF)-beta1 and tumor necrosis factor (TNF)-alpha are thought to play important roles in the development of pulmonary fibrosis. We recently reported that TNF-alpha upregulates TGF-beta1 expression in primary mouse lung fibroblasts (MLFs), a key cell population in fibrogenesis. In the present study, we have investigated signal transduction pathways involved in TNF-alpha upregulation of TGF-beta1 in both primary MLFs and the Swiss 3T3 fibroblast cell line. Treatment of fibroblasts with TNF-alpha resulted in a significant increase in TGF-beta1 protein as measured by ELISA. The increase in protein was preceded by a 200-400% increase in TGF-beta1 mRNA detected by quantitative, real-time, reverse transcriptase-polymerase chain reaction. Western blot analysis showed that TNF-alpha activated the extracellular signal-regulated kinase (ERK) and inhibitors of the ERK-specific mitogen activated protein kinase (MAPK) pathway (PD98059 or U0126) blocked TNF-alpha induction of TGF-beta1 mRNA and protein. mRNA stability experiments showed that TNF-alpha increased the half-life of TGF-beta1 mRNA to more than 24 h compared to ~ 15 h in unstimulated cells. Expression of constitutively active MEK1 that selectively phosphorylates ERK was sufficient for TGF-beta1 mRNA stabilization in Swiss 3T3 fibroblasts. These results indicate that TNF-alpha activates the ERK-specific MAPK pathway leading to increased TGF-beta1 production in fibroblasts, primarily via a posttranscriptional mechanism that involves stabilization of the TGF-beta1 transcript.