By computer calculation with the use of atom-atom potentials a stereochemical model of the DNA-hydrate Na+ complex was obtained according to which the Na+(H2O)6 octahedrons are localized in the narrow groove of DNA with formation of a great number of van der vaals contacts and hydrogen bonds. The model explains why the C-form of DNA is stabilized in the Na-DNA complex which is formed in the presence of 80% methanol (v/v). A possibility of the existence of such complexes in vivo at the DNA regions surrounded with media of diminished water activity (the DNA-membrane complex, chromosomes, phage heads) is discussed.