The toxicity of naphthalene in rodents has been attributed to the reactive metabolites naphthalene-1,2-oxide (NPO), 1,2-naphthoquinone (1,2-NPQ) and 1,4-naphthoquinone (1,4-NPQ). Differences in the formation of these reactive metabolites in different species can shed light on the mechanism by which naphthalene exerts its toxicity. Protein adducts allow investigators to study the disposition of reactive metabolites that cannot be measured directly. We measured cysteinyl adducts of the above metabolites in hemoglobin (Hb) and albumin (Alb) from the blood of male Swiss Webster mice dosed with 1.56-200mg naphthalene/kg b.w. Levels of NPO adducts (designated as NPO1-Hb, NPO2-Hb, NPO1-Alb and NPO2-Alb) increased nonlinearly with the administered dose; levels of Alb adducts were higher than those of Hb adducts; levels of NPO1 adducts were higher than those of NPO2 adducts. Levels of NPQ adducts (1,2-NPQ-Alb, 1,4-NPQ-Alb, 1,2-NPQ-Hb and 1,4-NPQ-Hb) were lower than those of NPO. Although NPQ-Alb increased with doses above 12.5 mg naphthalene/kg body wt. (b.w.), levels of NPQ-Hb barely increased above the background levels within the dose range examined. The shapes of the dose response curves for total cysteinyl adducts (combined NPO and NPQ) in Hb and Alb were consistent with previous results of radiobinding experiments in naphthalene-dosed mice. Dose-specific levels of NPO-Alb remained essentially constant in mice over the dose range of 25-200 mg/kg b.w. while those of 1,2- and 1,4-NPQ-Alb diminished over this range. Comparing dose-specific levels of NPO-Alb in Swiss Webster mice with those published previously in F344 rats suggests that glutathione depletion in mice occurred at about 1/8th the administered dose previously observed in rats. This suggests that mice could be more susceptible than rats to the toxic effects of naphthalene due to more pronounced depletion of glutathione at a given dose.