Partial hepatectomy triggers a variety of biological phenomena, which culminate in regeneration of the liver mass. Hepatocyte proliferation is a major feature of the regenerating liver after partial hepatectomy. Previous studies in our laboratory suggested that hepatic matrix remodeling might be a prerequisite process for hepatocyte proliferation in the regenerating liver. In the present study we use immunohistochemical staining, Western blot analysis, and gelatin zymography to show that the inactive matrix metalloproteinases (MMPs), pro-MMP-2 and pro-MMP-9, are elevated at 30 minutes and activated at 6 to 12 hours and at 3 to 6 hours, respectively, after hepatectomy. Sham-operated livers did not show an increase in inactive pro-MMP-2 and pro-MMP-9 and did not contain active MMP-2 or MMP-9. To examine whether tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) is changed to regulate the activities of MMPs after partial hepatectomy, the level of TIMP-1 protein was analyzed by both immunohistochemistry and Western blot analysis. The level of TIMP-1 protein appears to increase by 6 to 18 hours, implying that an increase in TIMP-1 regulates activities of active MMP-2 and MMP-9. Taken together, these results suggest that hepatic matrix remodeling is mediated by activated MMPs, which contribute to modulation of the environment surrounding hepatocytes during rat liver regeneration.