The objective of the present study was to establish whether H2O2 alone or in the presence of UVA or UVB would give rise to formation of the oxidatively damaged DNA base 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) in cultured adult rat liver (ARL-18) epithelial cells. Hydrogen peroxide alone at 5 mM increased 8-oxo-dG levels by 42% of that of culture control. Compared to culture control, UVB exposure at a dose of 0.63 J/cm2 elevated 8-oxo-dG levels only 8.4%. In the presence of 5 mM H2O2 + UVB (0.63 J/cm2), 8-oxo-dG levels were elevated 155% above culture control suggesting a synergistic effect. A UVA dose of 10 J/cm2 did not elevate 8-oxo-dG levels above culture control. In the presence of 5 mM H2O2 plus UVA (12 J/cm2), 8-oxo-dG levels were elevated 310% above controls compared with an increase of 75.8% above control levels at the same dose in the absence of H2O2. These results reveal that both UVA or UVB can promote H2O2 generation of reactive oxygen species (ROS) in whole cells resulting in an increase in the formation of 8-oxo-dG, although the photodynamic generation of ROS from H2O2 occurs with a much higher efficiency in the presence of UVB. Our study also demonstrates that 8-oxo-dG can be generated in cellular DNA of whole cells exposed to H2O2 and UVA or UVB, indicating that the ROS generated in whole cell systems are long enough lived to migrate to the nucleus and cause DNA damage.