The release of unaltered bases from irradiated DNA, hydrated between 2.5 and 32.7 mol of water per mole of nucleotide (gamma), was investigated using HPLC. The objective of this study was to elucidate the yield of the four DNA bases as a function of dose, extent of hydration, and the presence or absence of oxygen. The increase in the yield of radiation-induced free bases was linear with dose up to 90 kGy, except for the DNA with gamma = 2.5, for which the increase was linear only to 10 kGy. The yield of free bases as a function of gamma was not constant in either the absence or the presence of oxygen over the range of hydration examined. For DNA with gamma between 2.5 and 15, the yield of free bases was nearly constant under nitrogen, but decreased under oxygen. However, for DNA with gamma greater than 15, the yield increased rapidly under both nitrogen and oxygen. The yield of free bases was described by a model that depended on two factors: 1) a change in the DNA conformation from a mixture of the A and C conformers in vacuum-dried DNA to predominantly the B conformer in the fully hydrated DNA, and 2) the proximity of the water molecules to the DNA. Irradiation of the inner water molecules (gamma less than 15) was less efficient than irradiation of the outer water molecules (gamma greater than 15), by a factor of approximately 3.3, in forming DNA lesions that resulted in the release of an unaltered base. This factor is similar to the previously published relative efficiency of 2.8 with which hydroxyl radicals and base cations induce DNA strand breaks. Our irradiation results are consistent with the hypothesis that the G value for the first 12-15 water molecules of the DNA hydration layer is the same as the G value for the form of DNA to which it is bound (i.e., the pseudo-C or the B form). Thus we suggest that the release of bases originating from irradiation of the hydration water is obtained predominantly: (1) by charge transfer from the direct ionization of the first 12-15 water molecules of the primary hydration layer and (2) by the attack of hydroxyl radicals generated in the outer, more loosely bound water molecules.