Protein content, RNA concentration, in vivo and in vitro incorporation of L-leucine-14C (U.L.) into protein and of uridine-2-14C into RNA of the homogenate, and various fractions of muscle of mice were measured at various ages. Protein content showed a continuous increase in the homogenate, nuclei–myofibrillar, and soluble fraction of the muscle of mice undergoing development. Maximum increase was noted in the soluble fraction. RNA content dropped rapidly in the homogenate and in all subcellular fractions of the muscle up to 30 days, followed by a slow decline. Ratio of DNA as well as RNA content to protein decreased continuously in the homogenate and in all subcellular fractions with aging.In vivo incorporation of L-leucine-14C (U.L.) into protein of the homogenate and various subcellular fractions of the muscle decreased with the increasing age of the animal. In vitro incorporation of labelled leucine into protein by the cell-free preparation of skeletal muscle also decreased with aging. Such a decrease in the incorporation was thought to be due to decrease in the content of polysomes in the aging muscle.In vivo incorporation of uridine-2-14C into RNA of the homogenate and subcellular fractions of muscle showed a decrease with age. Similar types of results were obtained in studies in vitro.