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[Part 1]. For a series of PMR studies of cytidine and related compounds in D₂O solution at various pD, it was observed that the H₅ resonances of cytosine base are broad in some of these compounds at pD 4-7, while those of H₆ are not. We proposed that there is chemical exchange between two tautomeric forms of cytosine base to give rise to broadening of spectra. This chemical exchange was found to be catalyzed by both solvent (D₂O)and hydroxyl (OD⁻) ion present in solution. The observation of sharp resonances in certain cytidine derivatives having only one possible tautomeric structure supports this proposal. Temperature study of H₅ resonances at different pD was used to investigate the kinetics of this exchange reaction. The percentage of its minor tautomer was estimated as 10 ± 2% in neutral aqueous solution and its life time is about 10⁻³ sec at room temperature. The activation energy of this tautomeric exchange is 7.5 ± 0. 5 kcal/mole. for D₂O catalysis and 2.5 ± 0.5 kcal/mole for OD⁻ catalysis. The relation between tautomeric exchange of nucleic acids and genetic mutation is also discussed. [Part 2]. From the PMR study of guanosine and its related compounds at different pD, temperature and concentrations, it was observed that H₈ resonances of guanine base in some of these compounds are unusually broad over a wide pD range (from 2 to 8) in aqueous solutions. This phenomenon is explained in terms of the effects of base pairing, stacking and tautomeric exchange between the 1 and 6 positions of guanine base. The observation of sharp resonance of H₈ of 1-methyl guanosine having only one possible tautomeric structure supports that there exists minor tautomer of this base. This tautomeric exchange is catalyzed either by solvent (D₂O), base (OD⁻) or pairing of base. The percentage of minor tautomer and the activation energy of the exchange are also estimated from the kinetic analysis.