The most stable tautomers and rotamers of guanine are characterized by post Hartree-Fock ab initio calculations at the MP2 and CCSD(T) levels employing extended basis sets. Four of the lowest-energy structures (7H-oxo-amino- < 9H-oxo-amino- < 9H-syn-hydroxo-amino- < 9H-anti-hydroxo-amino-guanine) are found to lie within ca. 1 kcal/mol. The next form of guanine is established to lie more than 3 kcal/mol higher than the global minimum tautomer. The transition states of the following reversible reactions: 9H-oxo-amino-guanine right harpoon over left harpoon 9H-syn-hydroxo-amino-guanine right harpoon over left harpoon 9H-anti-hydroxo-amino-guanine have been studied. The calculated energy data were used to obtain thermodynamic parameters and to estimate the composition of the equilibrium mixture of conformers at 0 K and room temperature. The rate constants for the tautomerization of 9H-oxo-amino-guanine were determined by using the instanton approach. Their predicted values characterize an extremely slow chemical process, which is expected to reach the equilibrium in ca. 2500 h. Despite being so slow, we have shown that such a rate of the tautomerization describes a process that is much faster than the one characterized by the observed frequency of spontaneous point mutations. Therefore, additional stabilization factors, e.g., hydrations and interactions with enzymes are necessary to secure the known fidelity of DNA synthesis.