The enthalpies of the guanidinium chloride (Gu.HCl) with sodium DNA salt in the solutions in B- and A-conformations in the mixtures of ethanol-water at 298.15 K and the enthalpies of solution of guanidinium chloride in the mixtures of ethanol-water at 298.15 K in a whole range of the compositions of mixed solvents were measured calorimetrically. It was established that in a field of B-A-transition of DNA the values of interaction enthalpies of Gu.HCl with DNA practically do not depend on the composition of the solvent. The concentrations of Na-ions in water-ethanol solutions of DNA containing Gu.HCl were determined by the potentiometric method. It was revealed that the interaction of the equimolar quantities of Gu.HCl and DNA leads to the complete replacement of Na-cations, which are naturally linked with DNA, into solution. From the results obtained the enthalpy of B-A-conformation transition DNA at 298.15 K was determined (-2.50 +/- 0.10 kJ/mole).