The possible role of reactive oxygen species in the toxicity of smokeless tobacco (ST) was explored. The effects of an aqueous smokeless tobacco extract (STE) at doses of 125, 250 and 500 mg STE/kg in rats on the induction of hepatic mitochondrial and microsomal lipid peroxidation and the incidence of hepatic nuclear DNA damage 24 hours post treatment were examined. Dose-dependent increases of 1.8, 2.3 and 4.4-fold in mitochondrial and 1.5, 2.1 and 3.6-fold in microsomal lipid peroxidation occurred at 125, 250 and 500 mg STE/kg, respectively, relative to control values. At these same three doses of STE, 1.3, 1.4 and 2.7-fold increases in hepatic DNA single-strand breaks occurred relative to control values. STE administration also resulted in significant increases in excretion of urinary metabolites. Urinary excretion of the four lipid metabolites malondialdehyde (MDA), formaldehyde (FA), acetaldehyde (ACT) and acetone (ACON) was monitored by HPLC for 72 hours after treatment of rats with 125 and 250 mg STE/kg. Increases occurred in the excretion of the four lipid metabolites at every dose and time point with maximum increases in the excretion of all lipid metabolites being observed between 12 and 24 hours post treatment. The results suggest the involvement of an oxidative stress in the toxicity of STE.