The effects of losartan, an angiotensin type 1 receptor (AT1R) antagonist as (a pretreatment and combined treatment with bleomycin) on bleomycin induced rat lung fibrosis were evaluated & compared using light microscopy& immunohistochemical technique. Sections were taken from rats (n=56, 5 groups) at the 14th and the 28th day of the experiment. Sections were stained with Hematoxylin and Eosin, Masson's trichrome, Orcein and immunohistochemichal stains for α-SMA. The mean area percent of collagen & elastic fibers and mean area percent & optical density of immunopositive cells were measured as well as the number of pneumocytes type II was counted. Both cases of pretreatment with losartan at the dose of 10 mg/kg per day for 2 days before bleomycin administration and throughout the experiment as well as combined treatment of losartan with bleomycin at the dose of 10 mg/kg per day throughout the experiment showed significant reduction in fibrotic changes of the lung in bleomycin induced pulmonary fibrosis. However the pretreatment results were more significant than the combined treatment results.