The origin of meridional reflections in the X-ray diffraction patterns of vertebrate skeletal muscles in resting and rigor states was studied. The main results may be summarized as follows. 1. Most of the meridional reflections localized in groups at the positions of successive orders of the repeat period of about 430 A are contributed mainly by the C-protein component of thick filaments. 2. The meridional reflections at about 143 and 72 A in the X-ray diffraction pattern of the resting muscle are contributed mainly by the cross-bridge axial repeat period, while in the X-ray diffraction patterns of the rigorized muscle the reflections at approximately the same positions are contributed mainly by C-protein. The change in the positions of these particular reflections accompanying the transition of the muscle from rest to rigor and from rest to contraction cannot be considered as an indication of a change in the axial repeat period of the cross-bridges, as it was earlier suggested by some authors. 3. The transition of the muscle from resting to rigor state is accompanied by substantial changes in the positions of the meridional reflections contributed my minor proteins, which is indicative of the structural transition in the thick filaments. The observed changes may be interpreted as the result of the thick filaments elongation by about 1.5% or, alternatively, as a consequence of the redistribution of electron density of the meridional reflections 215 and 143 A during a single twitch of the muscle (Huxley et al., Nature, 1980 284, 140) may be interpreted as a natural consequence of the structural change in the thick filaments. It is concluded therefore that on stimulation of the vertebrate skeletal muscle the thickness filaments undergo a reversible structural change which may reflect the existence of myosin-linked regulation in that type of muscle.