To elucidate hepatic collagen metabolism during liver regeneration after partial hepatectomy, we measured collagen content, collagen synthesis, and collagen-degrading enzyme activity in the remnant livers of rats 3, 5, 7, and 14 days after a partial hepatectomy of 68%. Hepatic collagen synthesis was significantly higher 3, 5, and 7 days after partial hepatectomy than it was in sham-operated control rats, but there was no such difference 14 days after surgery, the maximal hepatic collagen synthesis being observed 5 days after surgery. Although the collagen concentration in the remnant liver was similar to that in the control liver, the total collagen content of the remnant liver increased rapidly with liver increased rapidly with liver regeneration until 7 days after partial hepatectomy. Hepatic collagenase activity was similar to the control; however, hepatic cathepsin B and cathepsin L activity and the intracellular degradation of newly synthesized collagen were markedly decreased 3, 5, and 7 days after partial hepatectomy compared with the controls. Hepatic collagen synthesis was significantly and inversely correlated with cathepsin L activity and with the intracellular degradation of newly synthesized collagen. These findings suggest that a combination of increased collagen synthesis and decreased intracellular collagen degradation contributes to the rapid supply of collagen that is observed during the early phase of liver regeneration. (Hepatology 1995;21:155–161).