DNA is subject to constant oxidative damage from endogenous oxidants. The oxidized DNA is continuously repaired and the oxidized bases are excreted in the urine. A simple routine analytical procedure is described for urinary 8-hydroxy-2'-deoxyguanosine, an oxidative DNA damage adduct, as an indicator of oxidative damage in humans and rodents. This adduct was purified from human urine and characterized. The described assay employs a series of solid-phase extraction steps that separate 8-hydroxy-2'-deoxyguanosine from other urinary constituents, followed by analysis by gradient reversed-phase HPLC coupled to a dual-electrode high-efficiency electrochemical detection system. Analysis of urine from three species by this method indicates that mice excrete approximately 3.3-fold more 8-hydroxy-2'-deoxyguanosine than humans (582 vs. 178 residues per cell per day), a result that supports the proposal that oxidative damage to DNA increases in proportion to species-specific basal metabolic rates.