The structure and energetics of the base pairing and proton transfer in the Watson-Crick type of guanine-cytosine base pair are studied by the HF/MINI-1 method within the full gradient optimization and self-consistent reaction field techniques. Two minima and the transition state on the connecting minimum energy path were located on the potential energy surface (PES) of this complex. The minima involved the canonical amino-keto base pair (GC) and the 4-iminocytosine-6-hydroxyguanine base pair (G*C*), which the latter being formed from GC by a simultaneous transfer of two protons in neighbouring hydrogen bonds. A polar environment was found to stabilize the canonical structure by 1·3 kcal mol-1 with respect to the rare tautomer G*C*. The GC pair represents the global minimum on the HF/MINI-1 PES in both gas phase and polar environment. The polar environment was also found to decrease the interaction enthalpy of the GC base pair and to influence significantly the geometry of the hydrogen bonds. The tautomeric origin of spontaneous point mutations and the recent observation of the different rotation rates about the cytosine and guanine amino bonds in the GC base pair are discussed.