The epoxidation of the gaseous alkene 2-methylpropene or isobutene was studied in vitro in rat lung tissue in comparison with rat liver. Pulmonary tissue appears to be less exposed to the toxic epoxide metabolite than is the case for hepatic tissue. The results are correlated with the low capacity of the mixed function oxidase system, expressed by means of the cytochrome P-450 content and the 7-ethoxycoumarin O-deethylase activity, to form reactive intermediates. The activities of the principal epoxide detoxifying enzymes glutathione S-transferase and epoxide hydrolase represent only 5-10% of the values measured in rat liver.