The reaction mechanism of calcium ion in the postmortem weakening of 2-disks was studied in myofibrils prepared from fresh or stored muscles. The a-actinin showing very limited proteolysis of myofibrils during postmortem storage of muscles at 10. The postmortem weakening of Z-disks was markedly dependent on muscle pH, showing a minimum at pH 6.5. These results agree well with calcium-induced weakening of Z-disks of freshly isolated myofibrils, indicating that no protease participates in the postmortem weakening of 2-disks. Z-disks of myofibrils prepared from stored muscles split into halves after treatment with 0.1N NaOH for 5min. The identical splitting of Z-disks was induced by a calcium ion concentration of 10^{-4}M, which is of the Same order of magnitude as that in the sarcoplasm in postmortem muscle. We therfore conclude that the postmortem weakening of Z-disks is non-enzymatically induced by the raised sarcoplasmic calcium ion concentration of 10^{-4}M. Calcium ions probably solubilize the amorphous cementing material of Z-disks, leaving unchainged the two sets of 2-filaments composed of α-ac-tinin.