To study the interaction of the fac-[M(CO)(3)](+) moiety (M = Tc-99m, Re-188) with DNA bases, we reacted [M(OH2)(3)(CO)(3)](+) with 9-methylguanine (9-MeG), guanosine (G), and 2-deoxyguanosine (2dG). Two bases bind to the metal center via the N7 atoms. X-ray structure analysis of [Tc-99(CH3OH)(9-MeG)(2)(CO)(3)](+) (4) (monoclinic, I2/a, a = 28.7533(14) Angstrom, b = 8.0631(4) Angstrom, c = 32.3600(15) Angstrom, beta = 91.543(6)degrees, V = 7499.6(6) Angstrom(3), Z = 8) and [Re(OH2)(9-MeG)(2)(CO)(3)](+) (7) (monoclinic, P2(1)/n, a = 12.2873(11) Angstrom, b = 16.0707(13) Angstrom, c = 14.1809(16) Angstrom, beta = 103.361(12)degrees, V = 2724.4(5) Angstrom(3), Z = 4) reveals that the two bases are in a head-to-tail (HT) orientation. Kinetic studies show that the rates of substitution of the purine bases are comparable to that of one of the active forms of cisplatin. The bis-substituted complexes are generally less stable than the platinum adducts, and metalation of the bases is reversible.