The effects of trinitrophenylation of lysyl residues of rabbit skeletal myosin subfragment 1 (S1) on thermal denaturation of S1 in the absence of nucleotides, in the presence of ADP and within S1 complexes with ADP and Pi analogues, orthovanadate (Vi) or beryllium fluoride (BeFx), have been studied by differential scanning calorimetry. It has been shown that lysyl trinitrophenylation significantly affects the thermal stability of S1, changes its domain structure, promotes the decomposition of S1.ADP.Vi and S1.ADP.BeFx complexes, and strongly prevents the structural changes in the S1 molecule induced by the formation of the S1.ADP.Vi complex without any effect on the thermal stability of S1 within S1.ADP and S1.ADP.BeFx complexes. It has been demonstrated that the effects of trinitrophenylation on the S1 structure are mainly due to specific modification of the epsilon-amino group of the Lys-83 residue.