The primary reaction in Ca-regulation of activity of skeletal muscle is the reception of the Ca2+ ion by troponin, followed by activation of thin filament. The calciumreceptive site of troponin lies in one of its subunis, TN-C. When muscle is relaxed at concentrations of Ca2+ ion less than 10-7M, TN-C is devoid of calcium ion and the activity of thin filament is suppressed by the action of TN-I, another subunit of troponin. Thus the function of TN-C is, by sensing the signal of calcium ion, to release the activity of thin filament from suppression exerted by TN-I. This is the essential pattern of the Ca-regulation, which has been revealed since the discovery of troponin by Ebashi and his colleagues. A large body of experimental results and discussions on this subject was presented at the Cold Spring Harbor Symposium on Quantitative Biology held in 1972. At the time of finding troponin, Ebashi postulated the occurrence of a Ca-induced conformation change of thin filament underlyling the activation of thin filament, which prompted research on the detailed structure and function of contractile elements and on the dynamics of their coupled functioning. In this special issue, several topics are chosen from works presented since the '72 CSH Symposium which indicate the present status of the field. Included are the following articles: (1) KITAZAWA, T. (Department of Pharmacology, Faculty of Medicine, Tohoku University) The role and movement of calcium ion in the cycle of contraction-relaxation of muscle. (2) WAKABAYASHI, T. (Department of Physics, Faculty of Science, University of Tokyo) On the quarternary structure of thin filament of muscle. (3) MIHASHI, K. and MIKI, M. (Department of Physics, Faculty of Science, Nagoya University) Conformation change of thin filament of skeletal muscle studied with fluorescence spectroscopy. (4) UENO, H. and OOI, T.* (Department of Physics, Faculty of Science, Nagoya University and *Institute of Chemical Research, Kyoto University)Structure of regulatory proteins; Mode of binding of tropomyosin and troponin. (5) IIDA, S. (Department of Physics, Faculty of Science, Nagoya University) Binding of calcium ion to troponin. (6) IIO, T. (Department of Physics, Faculty of Science, Nagoya University) Kinetic molecular mechanism of conformation change of troponin-C. (7) YAZAWA, M. and YOSHIDA, M. (Department of Cheminstry, Faculty of Science, University of Hokkaido) Myosin-linked regulation of muscle contraction.