The level of functional mRNA coding for myofibrillar proteins was studied during development of the chicken skeletal muscle. RNA isolated from the developing chicken muscle directed protein synthesis in a wheat germ cell-free system. By means of polyacrylamide gel electrophoresis and immunological analysis, tropomyosin subunits and troponin components were identified among the cell-free translation products. The mRNA activities for alpha- and beta-subunit of tropomyosin were prominent in the embryonic breast muscle as well as in the embryonic leg muscle. At the early post-embryonic stage, the mRNA activity for beta-subunit disappeared from the breast muscle, while those for alpha- and beta-subunit were detectable in the leg muscle. Troponin-C and troponin-I synthesized in vitro in response to the muscle RNA formed a binary complex in the presence of calcium ion. Despite the observed difference in molecular weight between troponin-Ts in the breast and leg muscle, RNA preparations from the two muscles encoded identical troponin-Ts whose molecular weights were indistinguishable from that of troponin-T present in the breast muscle of adult chicken. It is suggested from these results that the biosynthesis of tropomyosin is regulated at the pre-translational level during the development of the chicken skeletal muscle, whereas post-translational (or co-translational) events may produce the tissue-specific form of troponin-T.