Polarographie reduction of adenine and cytosine moieties in denatured DNA was followed by means of differential (derivative) pulse polarography. The measurements of the reduction currents yielded by samples of denatured DNA differing in guanine+cytosine (G + C) content showed that both cytosine and adenine residues were reduced in DNA in the pH range of 6.0 -8.7. The contribution of their reduction to the total reduction current of denatured DNA differs in the range pH 6.0 -8.7, however; the contribution of the current of adenine residues to the current of DNA decreases with increasing pH as compared with the contribution of cytosine residues. It was also shown that in the region of neutral pH reduction current of denatured DNA is independent of the G + C content in DNA.