Faradaic currents produced by DNA at neutral pH are studied by square-wave voltammetry (SWV) in connection with an HMDE. Under these conditions single-stranded DNA produces a cathodic peak (due to the reduction of adenine and cytosine residues) which is better developed than the peaks obtained by CV and DPV; the SWV peak can thus be measured at substantially lower DNA concentration (down to tens of nanograms per milliliter). The anodic peak G, which is due to oxidation of the guanine reduction product (formed at potentials around - 1.85 V against an AgAgCl3 M KCl electrode), is studied in greater detail. At potentials more negative than - 1.85 V a deeper reduction of the guanine residue takes place which results in a decrease of peak G and in changes of the peak potential. The dependences of the peak G on the pulse amplitude and square-wave frequency are unusual. Using adsorptive stripping transfer voltammetry it is possible to determine DNA in the presence of an excess of guanosine. Methylation of the DNA guanine residues by dimethyl sulfate results in a decrease of the peak G. A DNA-modified electrode can be used as a sensor for the DNA damage by methylating agents.