Ascorbic acid and other reducing agents can hydroxylate deoxyguanosine (dGuo) at the C-8 position in the presence of oxygen. The Udenfriend system, consisting of ascorbic acid, Fe²⁺, EDTA, and O₂, is effective for this reaction. Hydroxylamine, hydrazine, dihydroxymaleic acid, sodium bisulfite, and acetol also promote hydroxylation. Metal-EDTA complexes (Fe²⁻, Sn²⁻, Ti³⁻, Cu¹⁻) are also effective. The OH radical is likely involved in most hydroxylating systems, but other mechanisms may also be involved, especially when ascorbic acid or ascorbic acid plus H₂O₂ is used. The hydroxylation of guanine residues in DNA may have biological significance related to mutagenesis and carcinogenesis. The study shows that ascorbic acid can hydroxylate guanine residues in DNA, and that this modification may contribute to DNA damage and mutations. The hydroxylation of guanine residues in DNA may be involved in mutagenesis and carcinogenesis, as many reducing agents and metals used in this study are mutagenic or carcinogenic. The study also shows that ascorbic acid can induce reverse mutations in S. typhimurium and chromosome aberrations in human fibroblast cells. Ascorbic acid is also reported to have cocarcinogenic and tumor promoting activities. The hydroxylation of guanine residues in DNA may lead to structural and conformational changes that increase the likelihood of mispairing during DNA replication or DNA repair. The biological significance of hydroxylation of guanine residues in DNA is discussed.Ascorbic acid and other reducing agents can hydroxylate deoxyguanosine (dGuo) at the C-8 position in the presence of oxygen. The Udenfriend system, consisting of ascorbic acid, Fe²⁺, EDTA, and O₂, is effective for this reaction. Hydroxylamine, hydrazine, dihydroxymaleic acid, sodium bisulfite, and acetol also promote hydroxylation. Metal-EDTA complexes (Fe²⁻, Sn²⁻, Ti³⁻, Cu¹⁻) are also effective. The OH radical is likely involved in most hydroxylating systems, but other mechanisms may also be involved, especially when ascorbic acid or ascorbic acid plus H₂O₂ is used. The hydroxylation of guanine residues in DNA may have biological significance related to mutagenesis and carcinogenesis. The study shows that ascorbic acid can hydroxylate guanine residues in DNA, and that this modification may contribute to DNA damage and mutations. The hydroxylation of guanine residues in DNA may be involved in mutagenesis and carcinogenesis, as many reducing agents and metals used in this study are mutagenic or carcinogenic. The study also shows that ascorbic acid can induce reverse mutations in S. typhimurium and chromosome aberrations in human fibroblast cells. Ascorbic acid is also reported to have cocarcinogenic and tumor promoting activities. The hydroxylation of guanine residues in DNA may lead to structural and conformational changes that increase the likelihood of mispairing during DNA replication or DNA repair. The biological significance of hydroxylation of guanine residues in DNA is discussed.