Liver collagen levels are determined by a balance between synthesis and degradation, processes known to have rapid rates in growing animals. We report age-related changes in liver collagen synthesis and degradation rates, as well as protein synthesis rates, in rats at five ages from 1 to 24 mo. Fractional collagen synthesis rates were determined after injection of [14C]proline with a flooding dose of unlabeled proline and its incorporation as hydro~y-['~C]proline into proteins. Fractional protein synthesis rates were based on the uptake of [14C]proline into proteins. Fractional collagen degradation rates were calculated from the difference between collagen fractional synthesis and deposition rates. Fractional rates of collagen synthesis were similar between 1 m o (23.0% f 4.6%/day) and 24 mo (19.6% f 3.4%/day) of age. Collagen deposition into the extracellular matrix was extremely low at every age studied; therefore degradation pathways accounted for the bulk of the collagen synthesized. The mean fractional synthesis rate for the total protein pool was unaltered between 1 mo (105.0% & 7.2%/day) and 15 mo (89.9% f G.O%/day) of age, after which it increased to 234.9% f 33.O%/day (p < 0.05) by 24 mo of age. These results indicate that liver collagen and total protein synthesis rates were maintained at relatively high levels during development and maturity but that protein synthesis rates were highest in senescent animals. (HEPATOLOGY 1991;14:1224-1229.) Collagens of various types are the most abundant proteins in man and total approximately one third of body protein (1, 2). In liver they are found in the extracellular matrix and basement membranes, where their primary role is to provide a supportive extracellular framework for cells (3). Other functions of collagens include platelet adhesion and aggregation, cell attachment, chemotaxis, filtration in basement mem-