The　influence　of　binding　of　cisplatin　adducts　on　tautomeric　equilibrium　of　guanine　was　investigated　using　quantum　chemical　method.　The　monoaqua　adduct　[Pt(NH3)(2)CI(H2O)](+)　and　the　diaqua　adduct　[Pt(NH3)(2)(H2O)(2)](2+)　were　chosen　for　coordination　to　the　N(7)　site　of　guanine　tautomers.　The　results　demonstrate　that　the　platinum　adducts　influence　moderately　on　tautomeric　equilibrium,　but　do　not　change　the　relative　stability　of　tautomers　whether　in　gas　phase　or　in　aqueous　solution.　The　keto　form　having　H　atom　at　N(l)　and　N(9)　was　always　the　predominant　structure　when　cisplatin　adducts　were　bound　to　guanine.　However,　other　forms　could　coexist　in　water.　Meanwhile,　our　calculations　suggest　that　the　tautomeric　equilibrium　should　be　via　the　same　intermediate.