An investigation of the potential energy surface (PES) of guanine was performed. The (nonplanar) molecular geometries of seven different forms of guanine were studied using the ab initio LCAO-MO method at the MP2 level with valence double-zeta basis sets augmented by d- and p-polarization functions (6-31G(d,p)). Among the studied forms are the four lowest energy tautomers, one rotamer, and two transition state forms for proton transfer between oxo and hydroxo tautomers of guanine N(7)H and N(9)H tautomers. Our best estimation of the relative stabilities of the tautomers includes electron correlation contributions calculated at the MP2/6-31G(d,p) reference geometries at the MP4(SDTQ)/6-31G(d,p), MP4(SDQ)/6-311G(d,p), and MP2/6-311++G(df,pd) levels. Three tautomers (and one rotamer) are located within 10 kJ mol(-1) on the MP2 level PES of guanine. The calculated energy barriers for the proton transfer are larger than 150 kJ mol(-1) and do not change by more than a few kJ mol(-1) upon application of the SCRF model based on the Onsager approximation.