We have shown, using the proline:ornithine dual label method, that in normal rats, hepatocytes contribute in vivo about 80 to 90% of the newly synthesized hepatic collagen. In order to quantify the contribution of hepatocytes and nonparenchymal cells to collagen synthesis in vivo in hepatic fibrogenesis, rats with CCl4-induced liver fibrosis were given [5(3H)]proline and [14C]ornithine intraperitoneally. About 80% of the 14C in albumin and transferrin was present as arginine, following conversion of [14C]ornithine via the urea cycle. In contrast to hepatocyte proteins, in nonparenchymal cells and serum a negligible percentage of the radioactivity was present as [14C]arginine. These combined findings indicate that, in spite of the hepatocellular damage, the labeling of hepatocyte proteins was efficient and specific, validating the use of the proline:ornithine method in this experimental model of hepatic fibrosis. We calculated the [3H]proline/[14C]arginine ratio in hepatic collagen (after correcting for the relative frequencies of amino acids) as a percentage of the same ratio in either albumin or transferrin, the index hepatocyte proteins. In this experimental model, during active fibrogenesis, both hepatocytes and nonparenchymal cells increase their production of collagen 2-fold when compared to normal animals, and hepatocytes produce the majority of the newly synthesized hepatic collagen.