The relation between polarographic reduction, values of half-wave potentials, and the parameter of potential carcinogenicity (tg alpha) was studied in a series of synthetic analogs of natural nucleic acid components modified by different substituents at position 5 of the base. The series included pyrimidine nucleobases (5-cyanouracil, 5-carboxycytosine, 5-azacytosine), ribofuranosyl nucleosides (5-cyanouridine, 5-carboxycytidine, 5-azacytidine) as well as 2'-deoxy-5-azacytidine and 5-carboxy-1-beta-D-arabinosylcytosine. No direct correlation was found between the reducibility of the studied compounds and the parameter of potential carcinogenicity tg alpha since event a slight alteration in the structure of these molecules markedly affected the properties of these compounds.