In situ photolysis at 20 degrees C (argon plasma light source, lambda approximately greater than 200 mm) of oxygen-free solutions containing 2 mM H2O2 and heat-denatured, single-stranded (ss)DNA from calf-thymus resulted in the ESR spectra of the 6-hydroxy-5,6-dihydro-thymin-5-yl (1) and 5-methyleneuracil (3) radicals linked to the sugar-phosphate backbone. They were generated by reaction of OH radicals with DNA. By comparison of the decay characteristics of the ESR signals with rate constants from pulse-conductivity measurements [E. Bothe, G.A. Qureshi and D. Schulte-Frohlinde, Z. Naturforsch., 38c, 1030, (1983)] the thymine-derived radicals (1) and (3) can be excluded as precursors of the fast, dominating component of strand breakage of ssDNA. In the absence of H2O2 from native, double-stranded (ds)DNA an ESR signal was obtained (singlet, g approximately 2.004, delta v1/2 approximately 0.8 mT) which was assigned to the deprotonated guanine radical cation, [G.(-H)] of a DNA subunit. It is assumed that by the UV irradiation the guanine radical cation, (G+.), is generated, either by monophotonic photoionization or by electron transfer to pyrimidine bases. By rapid transfer of the bridging proton from (G+.) to the hydrogen bonded cytosine [G.(-H)] is formed. When photolysis of dsDNA was carried out in the presence of H2O2, reaction of photolytically generated .OH resulted in peroxyl radicals and purine radicals. The oxygen for formation of the peroxyl radicals is probably produced by reaction of [G.(-H)] with H2O2. Photolysis of N2O-saturated solutions containing dsDNA or ssDNA provided another possibility of generation of OH radicals. Under those conditions the .OH-induced radicals (1) and (3) were obtained not only from ssDNA but also from dsDNA.