The prototropic zwitterionic tautomerism of free adenine (Ade) and guanine ( Gua) was investigated by the semiempirical quantum-mechanical AMI method at the full optimization of structural parameters. This property ofpurine nucleotide bases was shown to be of molecular-zwitterionic features. Unlike pyrimidine bases, energetic intervals ofpurine molecular and zwitterionic tautomers are overlapped. Therewith zwitterionic tautomer with the most low energy is formed from molecular tautomeric ground state by the migration of the C8H carboproton to neighbour endocyclic N7 nitrogen atom, its relative energy being equal to 17,06 and 14,50 kcal/mol respectively for Ade and Gua.