Nicotine and smokeless tobacco extract have been shown to induce oxidative stress in different experimental systems. However, the effect of nicotine and smokeless tobacco extract containing equal amounts of nicotine on 8-OH-dG formation has not been investigated. 8-OH-dG is a DNA adduct formed by free radical attack and is elevated in tumor cells. The objective of the present study was to evaluate the formation of 8-OH-dG following exposure to different concentrations of nicotine and smokeless tobacco extract containing equal amounts of nicotine. Exposure of Chinese Hamster Ovary cells to 5 and 10 mM of nicotine resulted in generation of 8-OH-dG. The observed 8-OH-dG levels at these concantrations of nicotine were 5.1 +/- 1.7 and 9.4 +/- 2.3 8-OH-dG/dG x 100.000, respectively. Smokeless tobacco extract containing 1 and 5 mM nicotine also induced generation of 8-OH-dG. The measured 8-OH-dG levels at these concentrations were 4.7 +/- 0.6 and 20.6 +/- 2.2 8-OH-dG/dG x 100.000 respectively. Exposure of cells to smokeless tobacco extract containing 10 mM nicotine resulted in cell death. Coaddition of superoxide dismutase and catalase along with nicotine reversed the 8-OH-dG generation. However, superoxide dismutase and catalase did not inhibit the 8-OH-dG generation in smokeless tobacco extract treated cells.