SummaryStudies on the incorporation of DL‐[1‐14C] leucine into myosin, total myofibrillar protein and total sarcoplasmic protein have shown agedependent alterations in the rate of synthesis of these proteins in red and white skeletal muscles of chicks. During the early phase ofex ovodevelopment white muscle synthesizes significantly higher amounts of myofibrillar proteins, especially myosin, in comparison with red muscle. The rate of sarcoplasmic protein synthesis in red and white muscles one day after hatching is almost identical. The red muscle shows a markedly higher rate of sarcoplasmic protein synthesis from 10 days after hatching. The incorporation of amino acid into various protein fractions of both the muscle types decreases with advancing age. In adult chicks red muscle displays a higher ability to synthesize sarcoplasmic and myofibrillar proteins.