Using negative staining, freeze-drying, and shadowing techniques in electron microscopy we have for the first time demonstrated Ca-induced reversible structural transitions in the synthetic filaments of dephosphorylated column-purified rabbit skeletal and cardiac muscle myosins formed by dialysis against solutions containing 120 mM KCl, 1 mM MgCl2, 10 mM imidazole-HCl buffer (pH 7.0), and either 0.1 mM CaCl2 or 1 mM EGTA It has been revealed that the compact ordered structure of the filaments with myosin heads and subfragments-2 (S2) disposed close to the filament backbone with an axial periodicity of about 14.5 nm in the absence of Ca2+ transforms into a spread disordered structure due to the movement of the heads and S2 away from the filament surface in the presence of Ca2+. Increasing the pH from neutrality to pH 7.8 leads to a spread, disordered structure while decreasing the pH value to 6.5 returns the filaments to their compact, rather ordered state independent of the Ca2+ concentrations used. The fact that the reversible structural transitions in synthetic filaments of myosin are observed in the absence of actin and actin and myosin-associated proteins suggests that Ca2+-induced S2 movement is an intrinsic property of myosin itself. Ca2+-induced S2 mobility may reflect the existence of functionally significant communications between the myosin head domains and the tails of myosin molecules in thick filaments, and its disappearance can be an indicator of the impairment of these communications, for example, in acute ischemia and myocardial infarction.