Human [G(3)(TTAG(3))(3)] and Oxytricha [G(4)(T(4)G(4))(3)] telomere model oligonucleotides, PSO-1 and PSO-2, bearing two fluorophores, 6-carboxyfluorescein (6-FAM) and 6-carboxytetramethylrhodamine (6-TAMRA) at their 5'- and 3'-termini, respectively, were synthesized. Both of them can form an intramolecular antiparallel tetraplex upon addition of K(+), and an enhanced fluorescence resonance energy transfer (FRET) was observed. PSO-1 showed a 43,000 times higher selectivity for K(+) against Na(+). Fluorometric and circular dichroism spectrophotometric studies revealed that this system is useful for the evaluation of the interaction of different telomeric repeat oligonucleotide sequences with metal ions.