The aim of this study is to investigate the molecular mechanisms underlying delayed progressive pulmonary fibrosis, a characteristic of subacute paraquat (PQ) poisoning. Epithelial-mesenchymal transition (EMT) has been proposed as a cause of organ fibrosis, and transforming growth factor-ß (TGF-ß) is suggested to be a powerful mediator of EMT. We thus examined the possibility that EMT is involved in pulmonary fibrosis during PQ poisoning using A549 human alveolar epithelial cells in vitro. The cells were treated with various concentrations of PQ (0-500 µM) for 2-12 days. Short-term (2 days) high-dose (>100 µM) treatments with PQ induced cell death accompanied by the activation of caspase9 as well as a decrease in E-cadherin (an epithelial cell marker), suggesting apoptotic cell death with the features of anoikis (cell death due to the loss of cell-cell adhesion). In contrast, long-term (6-12 days) low-dose (30 µM) treatments with PQ resulted in a transformation into spindle-shaped mesenchymal-like cells with a decrease of E-cadherin as well as an increase of a-smooth muscle actin (a-SMA). The mesenchymal-like cells also secreted the extracellular matrix (ECM) protein fibronectin into the culture medium. The administration of a TGF-ß1 receptor antagonist, SB431542, almost completely attenuated the mesenchymal transformation as well as fibronectin secretion, suggesting a crucial role of TGF-ß1 in EMT-like cellular response and subsequent fibrogenesis. It is noteworthy that despite the suppression of EMT-fibrogenesis, apoptotic death was observed in cells treated with PQ+SB431542. EMT-like cellular response and subsequent fibrogenesis were also observed in normal human bronchial epithelial (NHBE) cells exposed to PQ in a TGF-ß1-dependent manner. Taken together, our experimental model reflects well the etiology of PQ poisoning in human and shows the involvement of EMT-like cellular response in both fibrogenesis and resistance to cell death during subacute PQ poisoning of pulmonary epithelial cells.