The aim of this study was to investigate whether transforming growth factor-h1 (TGF-h1) could induce alveolar epithelial to mesenchymal transition (EMT) in vitro. Alveolar epithelial cells (AECs) from SD rats were isolated by elastase cell dispersion and IgG panning. Expression of a-smooth muscle actin (a-SMA) was assayed using Western blotting and immunostaining analysis. Morphological changes, the markers of epithelial cell (E-cadherin), and stress fiber by actin reorganization were detected by an indirect immunostaining. The contents of collagen I were determined by spectrophotometry. The levels of endogenous TGF-h1 were measured with ELISA. Incubation of AECs with TGF-h1 (0.1～10ng/mL) induced abundant expression of a-SMA protein, and a-SMA expression in AECs reached a plateau when TGF-h1 was N 3ng/mL. Furthermore, we found that TGF-h1 (3ng/mL) exposure of AECs induced an authentic EMT characterized by abundant expression of a-smooth muscle actin, transformation of myofibroblastic morphology, increased formation of stress fiber by actin reorganization, and loss of epithelial marker E-cadherin. Meanwhile, significant increase in the levels of collagen I from 32.0 F 6.6mg/g in control to 98 F 10.8mg/g in TGF-h1-treated group was found over a 72h incubation period. Moreover, following stimulated by TGF-h1 (3ng/mL), a marked and time-dependent increase in endogenous TGF-h1 released from AECs was observed. At time points 72h, TGFh1 release mounted to 3451pg/ml, which was much enough to induce EMT in vitro. These results demonstrated that AECs, under stimulation of TGF-h1, underwent a conversion process into myofibroblasts in vitro.