Theoretical study of the "sandwich" models of the metal ion (Na+ and K+) G-tetrad complexes has been performed at the HF/6-311G(d,p)//HF6-31G(d,p) level. The distances between cations and the O6 atoms of,guanines have been predicted to be 2.826 Angstrom for sodium and 2.951 Angstrom for potassium in the G4-M-G4(inter) model and 2.791 Angstrom for sodium and 2.927 Angstrom for potassium in the G4-M-G4(intra) model. These results are consistent with the Na+-O6 distance of 2.75 +/-0.02 Angstrom in the corresponding crystal structures. The binding energy between the cations and the G tetrad in the tetraplexes is greater for Na+ than for K+. This suggests the domination of the electrostatic interaction in the cation-tetraplexe s systems. After the hydration energy correction, the stability sequence of monovalence cations in the guanine tetraplexes is found to be K+ > Na+. This finding gives the first direct theoretical evidence to support the hypothesis of Hud et al. that the preferred coordination of K+ over Na+ in G tetraplexes is actually dominated by relative energies of hydration.