Electronic structure calculations using both ab initio MO and DFT methods, in conjunction with the 6-311 ++G-(d,p), 6-311++G(3df,2p), and aug-cc-pVTZ basis sets, have been applied to investigate the energies and structures of the lowest-lying triplet states of the series of five parent nucleotide RNA and DNA bases including uracil, thymine, cytosine, guanine, and adenine and the halogenated 5X and 6X-substituted uracils (X = F, Cl, and Br). The singlet-triplet gap EST of uracil was evaluated using different functionals (B3LYP, B3P86, B3PW91, BP97-1, HCTH). MP2 and CCSD(T) methods were considered for uracil and thymine. For other bases, only B3LYP computations were performed. Computed results agree relatively well with those derived from recent electron impact study (Abouaf et al., Chem. Phys. Lett. 2003, 381, 486). For each base, the vertical triplet state is calculated at 3.5-3.8 eV above the corresponding singlet ground state but about I eV below the first excited singlet. Although geometrical relaxation of triplet structures involving out-of-plane distortions of hydrogen atoms leads to a stabilization of 0.7-0.8 eV, the triplet state is of ( pi* <-- 7) character. The DeltaE(ST) values are evaluated as follows (values in eV and referred to So): uracil, 3.63 (vertical, exp, 3.65 +/- 0.05)13.00 (adiabatic); thymine, 3.50 (exp, 3.60 +/-0.05)/2.85; cytosine, 3.60/3.09; guanine, 3.84/3.06; adenine, 3.71/3.09. For uracil, halogen substitution at C(5) tends to reduce the AEST value by up to 0.3 eV whereas C(5) methylation and C(6) halogenation induce only small changes. A vibrational analysis pointed out that the frequencies associated with the main normal modes such as C=O and N-H stretching motions are only slightly shifted upon excitation.