Matrix metalloproteinase-II (MMP-II, 72-kd type IV collagenase, or gelatinase) is one of the gene families of zinc enzymes capable of degrading extracellular matrix molecules, and specifically of degrading type IV and V collagens, gelatin, fibronectin, and elastin. In this study, we used both the liver fibrosis model and the reversibility model of experimental cirrhosis to clarify how MMP-II participates in liver fibrosis of rats. To produce fibrosis model, rats received subcutaneous injections of CCI4 twice weekly for 7, 9, or 14 weeks. For the reversibility model, rats were treated with CCl4 three a week for 8 weeks and killed at 3, 7, 14, 28, or 42 days after discontinuation of treatment. MMP-II gene expression was studied by Northern hybridization technique, and gelatinase activity of MMP-II was examined by zymography using gelatin substrate. At the same time, an immunohistochemical study using anti-type IV collagen antibody was carried out. In liver fibrosis model, nodule formation was established at 14 weeks. Immunodeposit of type IV collagen was increased in wide fibrous septa and was clearly observed along sinusoidal wall. Gene expression of MMP-II increased up to 7 to 12 times compared with that of controls, with the expression rate being maximum at an intermediate stage of fibrosis. Zymography showed the expressions of both 65-kd latent MMP-II, which is confirmed to be activated by adding p-aminophenylmercuric acetate, and 62-kd active MMP-II during fibrosis. The expression of both forms increased 13 to 28 times as the fibrosis progressed. By contrast, little latent MMP-II was detected in control livers. The percent active form to total MMP-II at each stage was elevated most at an intermediate stage of fibrosis up to 30% and decreased to 16% in the cirrhotic stage. As cirrhosis reversed, fibrous septa became thin but still persisted at 42 days in the reversibility model. Immunostaining of type IV collagen was increased in thin septa and faintly observed along sinusoid. Gene expression was elevated 18-fold and recovered gradually to remain elevated 18-fold and recovered gradually to remain elevated at 42 days after the discontinuation of intoxication. Expressions of both active and latent forms detected by zymography were elevated 15-fold during the early reversible stage and decreased gradually after the discontinuation of intoxication. These results indicated MMP-II may participate in pathogenesis of liver fibrosis and cirrhosis.