Summary The polarographic reduction of denatured DNA takes place in the adsorbed state. Reduction polarographic currents of denatured DNA in those pH regions, where the current does not change much with pH, have the character of adsorption currents under conditions when the surface of mercury electrode is fully covered by adsorbed polynucleotide molecules. If the surface is not fully covered, the currents in this pH region rather have the character of diffusion controlled ones. Desorption of denatured DNA at negative potentials inhibits the polarographic reduction current. This inhibition influences the usual shape of the d.c. polarographic curve so that a curve arises, which has a shape of single maximum or of double wave . The potential of denatured DNA desorption from a negatively charged electrode and, consequently, the shape of the d.c. polarographic curve depend on pH, type and concentration of salts, or on concentration of another substance capable of screening the charge of DNA phosphate groups, e.g. spermine.