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Background Idiopathic pulmonary fibrosis (IPF) is the most common and devastating form of interstitial lung disease (ILD) in the clinic.There is no effective therapy except for lung transplantation.Rapamycin is an immunosuppressive drug with potent antifibrotic activity.The purpose of this study was to examine the effects of rapamycin on bleomycininduced pulmonary fibrosis in rats and the relation to the expression of metalloproteinase-9 (MMP-9) and tissue inhibitor of metalloproteinase-1 (TIMP-1).Methods Sprague-Dawley rats were treated with intratracheal injection of 0.3 ml of bleomycin (5 mg/kg) in sterile 0.9％ saline to make the pulmonary fibrosis model.Rapamycin was given at a dose of 0.5 mg/kg per gavage,beginning one day before bleomycin instillation and once daily until animal sacrifice.Ten rats in each group were sacrificed at 3,7,14,28 and 56 days after bleomycin administration.Alveolitis and pulmonary fibrosis were semi-quantitatively assessed after HE staining and Masson staining under an Olympus BX40 microscope with an IDA-2000 Image Analysis System.Type Ⅰ and Ⅲ collagen fibers were identified by Picro-sirius-polarization.Hydroxyproline content in lung tissue was quantified by a colorimetric-based spectrophotometric assay,MMP-9 and TIMP-1 were detected by immunohistochemistry and by realtime quantitative reverse transcriptase polymerase chain reaction (RT-PCR).Results Bleomycin induced alveolitis and pulmonary fibrosis of rats was inhibited by rapamycin.Significant inhibition of alveolitis and hydroxyproline product were demonstrated when daily administration of rapamycin lasted for at least 14 days.The inhibitory efficacy on pulmonary fibrosis was unremarkable until rapamycin treatment lasted for at least 28 days (P ＜0.05).It was also demonstrated that rapamycin treatment reduced the expression of MMP-9 and TIMP-1 in lung tissue that was increased by bleomycin.Conclusion These results highlight the significance of rapamycin in alleviating alveolitis and pulmonary fibrosis,which is associated with decreased expression of MMP-9 and TIMP-1.