Conventional and saturation transfer electron paramagnetic resonance spectroscopy and differential scanning calorimetry were used to study the internal dynamics and stability of cardiac myosin. Intact and LC 2-deficient myosin isolated from bovine heart were spin-labelled with maleimide and iodoacetamide probe molecules at the SH1 sites. It was found that the probe molecules rotate with an effective rotational correlation time of 42 ns, which is at least six times shorter than the rotational correlation time of the same label on skeletal myosin. Addition of MgADP induces intrinsic changes in the multisubunit structure of myosin, but it does not lead to changes of the overall rotational properties of the myosin head. Temperature dependence of the EPR spectra of maleimide-labelled myosin shows continuous decrease of the spectral parameters (intensity ratio of the peak heights, hyperfine splitting) at increasing temperature. However, marked changes were obtained at about 16 degrees C in LC 2-deficient myosin. DSC measurements also support the view that the removal of the LC 2 light chain produces change in the internal structure of cardiac myosin.