Electron paramagnetic resonance (EPR) spectroscopy is used to study radical trapping in crystalline oligodeoxynucleotides exposed to 70 keV x-irradiation at 4 K and annealed to 240 K. The four oligomers studied were the Z form d(CGCACG:GCGTGC), two A forms, d(CCCTAGGG)(2) and d(GTGCGCAC)(2), and the B form d(CGCGAATTCGCG)(2). In each of these oligomers, evidence was found for trapping of a cytosine radical formed by the net gain of a hydrogen at C-6 and a proton at N-3 (the CYt(C-6+H, N-3+H+)(+circle) radical). The data are consistent with the trapping of another cytosine radical formed by the net gain of hydrogen at C5 (the CYt(C-5+H, N-3+H)(+circle) or Cyt(C-5+H)(circle) radical). The well-known thymine radical formed by the net gain of hydrogen at C5 (Thy(C-6+H)(circle) radical) was observed in the Z- and B-form duplexes but not in the A-form duplexes. The relative yields of these three reduction species indicate that cytosine is comparable to, or better than, thymine as a stable trapping site for reductive damage. These three radicals, CYt(C6+H, N-3+H+)(+circle), CYt(C-5+H, N3+H)(+circle), and Thy(C-6+H)(circle), account for similar to85% of the total irreversibly trapped electrons in samples irradiated at 4 K and annealed to 240 K. Extrapolation of these results to B-form DNA hydrated to 9 waters per nucleotide, x-irradiated at 4 K, and warmed to room temperature predicts end product yields of 0.04-0.06 mumol/J for 5,6-dihydrouracil and 0.03-0.05 mumol/J for 5,6-dihydrothymine.