To examine whether serum collagenase activity reflects the amount of collagenase activity in the fibrotic liver of rats, we simultaneously measured serum and hepatic collagenase activities in rats with carbon tetrachloride-induced liver injury. Serum collagenase was measured after reactivation by denaturing and dissociating the inhibitors with potassium thiocyanate and aminophenylmercuric acetate, while hepatic collagenase was measured after the removal of plasma protein and activation with aminophenylmercuric acetate. Serum and hepatic collagenase activities increased in the carbon tetrachloride-treated rats with the progression of liver fibrosis, and both activities of collagenase were closely correlated each other. These results suggest that serum collagenase activity measured in these assay conditions could be used as a noninvasive marker for hepatic collagenolysis in carbon tetrachloride-treated rats.