The present study was planned to investigate the ultrastructural changes and hydroxyproline level in the lungs of the mice induced. by bleomycin treatment. Following 4 weeks of bleomycin treatment, the lungs were examined histologically (using light and electron microscopes) and the hydroxyproline level was measured at 4th, 6th, 8th, and 10th weeks of the experiment. Biochemical and structural changes that normally occur in lung fibrosis were also observed in our study. For example; lung hydroxyproline level increased progressively (p<0.05) and structural changes advanced to interstitial fibrosis from 4th to 10th weeks of the experiment. Alveolar septum thickening, type II pneumocytes increase and mononuclear cell infiltration (lymphocyte, monocyte and macrophage) were seen. In addition, membrane-bound rod-shaped amorphous structures in the cytoplasm of alveolar macrophages were increased significantly (P<0.05).