The chapter discusses the effects of some chemical mutagens and carcinogens on nucleic acids. Consideration of the known chemical action of alkylating agents on deoxyribonucleic acid (DNA) suggested two principal possible mechanisms. First, anomalous base pairing in DNA could be envisaged to result from either ionization or tautomeric change of alkylated guanine. Second, removal of alkylated purines from the DNA template, by hydrolysis, could cause alterations in dual-energy x-ray absorptiometry (DXA) base sequence, possibly by the deletion of a base pair. The carcinogenic dialkylnitrosamines are shown to yield effective alkylating agents, by oxidative metabolism, and, as previously mentioned, these can alkylate both ribonucleic acid (RNA) and DNA in vivo. The alkylation of guanine residues in DNA can occur in mammalian cells in vivo following the administration of chemical carcinogens. Also, some quantitative correlation is emerging between the extent of this reaction and the incidence of tumors in the affected tissues. The possibility that degradation of RNA, as observed to follow from the action of diazomethane, may be a significant reaction in vivo that could account for their observation that molecular ribonucleic acid (mRNA) is lost from the ribosomes of dimethylnitrosamine treated rats.