Objective To study the pathology and possible mechanism of experimental hydrochloric acid（HCl） inhalation-indued pulmonary fibrosis in rats.Methods 120 male SD rats were randomly divided into a nomal control group，a bleomycin group，a high dose HCl group，a middle dose HCl group and a low dose HCl group.The bleomycin group was intratracheally injected with bleomycin once to induce pulmonary fibrosis.The three HCl groups were intratracheally injected with HCl once per week.The control group was given saline by the same way.Six rats of each group were randomly sacrificed on day 7，14，28 and 42 respectively.The histological changes of lung tissue were studied by HE and Masson’s trichrome staining.Hydroxyproline level in lung tissue was measured by digestion method.Protein and mRNA expression of transforming growth factor-β1（TGF-β1） were assayed by immunohistochemistry and RT-PCR respectively.Results Alveolitis in three HCl groups was significantl compared to control group，most severe at the second week，then remained at a high level which was equivalent to or exceeded the level of the bleomysin group after 28 days.Pulmonary fibrosis in three HCl groups was also significantly more severe than that in the control group，but milder than that in the bleomysin group.The high-dose and middle-dose HCl groups were not significantly different from the bleomysin group on day 42.There was no difference between three HCl groups in the earlier period，but the high-dose HCl group has a significantly difference from low-dose group on day 42.The content of hydroxyproline in high-dose and middle-dose HCl groups was also significantly higher than that in the control group.On day 42 hydroxyproline content in high-dose HCl dose rather middle –or low dose group was similiar with the level of bleomysin group.Content of TGF-β1 mRNA in three HCl groups was comparable to the level of bleomysin group on day 28 and exceeded on day 42.The expression of TGF-β1 in three HCl groups was not significantly different from the bleomysin group on day 42.Conclusion Experimental acid aspiration might contribute to pulmonary fibrosis in rats.Acid induced alveolar epithelial cell damage，abnormal proliferation and repair and fibrosis could be involved..