Naphthalene, a selective pulmonary bronchiolar cytotoxicant in the mouse, is metabolized in the liver to reactive metabolites that are capable of circulating and becoming bound irreversibly in extrahepatic tissues in vivo. Circulating reactive metabolites generated in the liver could interact with extrahepatic tissues either directly to produce toxicity or indirectly by depleting cellular defense capabilities. The studies reported here were to determine whether naphthalene oxide, an obligate and unstable intermediate in the metabolism of naphthalene, is capable of diffusing from intact hepatocytes. Efflux was measured by trapping the epoxide with [3H]glutathione followed by subsequent quantitation of the labeled glutathione adducts. Seventeen to 35% of the total amount of naphthalene oxide formed intracellularly was trapped extracellularly in 15- and 30-min incubations. The quantity of naphthalene oxide effluxing from isolated hepatocytes increased with increasing substrate concentrations. However, the relative amount of epoxide leaving the cell as a percentage of the total formed did not change over naphthalene concentrations ranging from 0.015 to 1.5 mM. Reactive naphthalene metabolites capable of binding covalently to extracellular proteins diffused from isolated hepatocytes in a time- and concentration-dependent manner. The ratio of extracellular to intracellular covalent binding was dependent upon the concentration of naphthalene in the incubation; at low naphthalene concentrations, covalent binding was higher extracellularly than intracellularly, whereas, at high concentrations, metabolites were predominantly bound intracellularly. These studies suggest that there is no threshold for the efflux of naphthalene oxide but that the relative amounts of reactive metabolite bound intra- vs. extra-cellularly may depend upon saturation of intracellular detoxication capabilities.