Objective To investigate the effects of mesenchymal stem cells (MSCs) on pulmonary fibrosis under different conditions (different types, different doses and different transplantation times in the same dose). Methods Seventy-two male SD rats were equally divided into 6 groups by random number table method: Group A received intratracheal instillation of NS (0.1 ml). Group B, C, D, E and F received intratracheal instillation of bleomycin (5 mg/kg in 0.1 ml NS). On the 1st day after bleomycin administration, Group A and B received NS (1.0 ml) via the tail vein, while Group C received bone marrow mesenchymal stem cells (BMSCs) 1×106, Group D received umbilical cord blood mesenchymal stem cells (UCB-MSCs) 1×106, Group E received BMSCs 5×105 and Group F received BMSCs 1×107 via the tail vein, respectively. On the 8th day after bleomycin administration, Group E received BMSCs 5×105 again. Half of rats in each group were sacrificed respectively on the 28th and 42nd day after bleomycin administration. The pathologic changes, transforming growth factor-β1 (TGF-β1), hydroxyproline (HYP), matrix metalloproteinase-2 (MMP-2) and tissue inhibitor of metalloproteinase-1 (TIMP-1) of the lung tissues were investigated. Results Alveolar structure was severely in disorder, and massive collagenous fiber was deposited in Group B. In Group B, C, D, E, F and A, the scores of pulmonary fibrosis were decreased in sequence [(3.00±0.00), (2.17±0.75), (1.60±0.89), (1.33±0.52), (1.00±0.00), (0.00±0.00) on the 28th day after bleomycin administration, and (3.00±0.00), (2.40±0.55), (1.75±0.96), (1.50±0.84), (-), (0.00±0.00) on the 42nd day after bleomycin administration], while the levels of TGF-β1, HYP and MMP-2/TIMP-1 were respectively decreased in sequence as above. The levels of TGF-β1, HYP and MMP-2/TIMP-1 were positively associated with pulmonary alveolitis and fibrosis scores respectively (P=0.000). Conclusion MSCs transplantation can suspend the progress of pulmonary fibrosis, and the mechanism may be related to reducing the level of TGF-β1 and improving the imbalance of MMP/TIMP. Choosing UCB-MSCs or multiple transplantation may improve the therapeutic effects of pulmonary fibrosis. Key words: Pulmonary fibrosis; Bleomycin; Bone marrow mesenchymal stem cells; Umbilical cord blood mesenchymal stem cells