We report a solid-state (23)Na NMR study of the bound sodium cations in a G-quadruplex formed by Oxytricha nova telomere DNA repeat, d(G(4)T(4)G(4)) (Oxy-1.5). Using a 2D multiple-quantum magic-angle spinning (23)Na NMR method, we observed three sodium cations residing inside the quadruplex channel of the Na(+) form of Oxy-1.5. Each of these sodium cations is sandwiched between two G-quartets. We found no evidence for sodium cations in the T(4) loop region. For comparison, solid-state (15)N MAS NMR spectra were also obtained for the (15)NH(4)(+) form of Oxy-1.5. The insufficient resolution in the (15)N MAS NMR spectra did not permit determination of the number of NH(4)(+) ions inside the quadruplex channel. The solid-state (23)Na and (15)N NMR spectra for Oxy-1.5 were also compared with those obtained for guanosine 5'-monophosphate.