Electron paramagnetic resonance (EPR) and differential scanning calorimetric (DSC) measurements were performed to study the motional dynamics and structural stability of myosins prepared from skeletal muscle of rabbit and cardiac myosin of bovine heart in different intermediate stares of the ATP pathway in solution and supramolecular complexes. ADP, the nonhydrolyzable ATP analogue 5’-adenylyl imidodiphosphate (AMP.PNP) and ADP plus orthovanadate (Vi) were used in different muscle model systems, as chemically skinned muscle fibres, myofibrils prepared from chemically skinned fibres and myosin solutions to simulate the pre-power (AMP.PNP-state, ADP.V-i-state) and post-power (rigor state, ADP-state) stroke conformations of myosin heads. Both DSC and EPR measurements support the view that the myosin heads following attachment to actin undergo a sequence of conformational states that differ both dynamically and energetically from each other. The differences in the melting temperatures and rotational mobility at the different intermediates clearly indicate significant alterations in the internal microstructure of myosin head region induced by binding of nucleotides.