The results of determination of apparent heat capacities of naturally occurring DNA in native (helix) and denatured (coils) states in dilute aqueous solutions have been reported. These results, supplemented by direct heat capacity measurements as a function of temperature, using a capillary scanning calorimeter, clearly show that the apparent heat capacity of polynucleotide chains of DNA in aqueous solution is strongly dependent upon the conformational state of the macromolecule. Comparison of data obtained for DNA in the "solid state", at different levels of hydration and for DNA in H2O and D2O solutions reveals the possibility that the heat capacity increment (Delta c(p) = 0.36 +/- 0.04 J g(-1) K-1) is determined by an increase in the number of vibrational degrees of freedom, by hydrophobic effects and as a result of hydrogen bond destruction in water (including the ordered water clusters in the hydration shells of double helix of DNA).