AIM: To investigate the effects of heme oxygenase-1(HO-1) against oxidant-induced injury caused by bile duct ligation (BDL).METHODS: Either cobalt protoporphyrin (CoPP), a HO-1 inducer, or saline were injected intraperitoneally in male SD-rats. Three days later, BDL or sham-operations were performed. Rats were sacrificed 3 wk after BDL and livers were harvested for histology. Fibrosis was evaluated by sirius red staining and image analysis.Alpha-smooth muscular actin, which indicates activation of stellate cells, was detected by immunohistochemical staining, and cytokine and collagen- Ⅰα (Col- Ⅰα) mRNA expression was detected using RNase protection assays.RESULTS: Serum alanine transaminase increased 8-fold above normal levels one day after BDL. Surprisingly,enzyme release was not reduced in rats receiving CoPP.Liver fibrosis was evaluated 3 wk after BDL and the sirius red-positive area was found to be increased to about 7.8%. However, in CoPP pretreated rats sirius redpositive areas were increased to about 11.7% after BDL.Collagen- Ⅰα and TGF-β mRNA increased significantly by BDL. Again, this effect was increased by HO-1overexpression.CONCLUSION: Hepatic fibrosis due to BDL is not reduced by the HO-1 inducer CoPP. In contrast, HO-1overexpression increases liver injury in rats under conditions of experimental chronic cholestasis.