A method for the determination of absorbed dose of ozone (03) in animals and humans using oxygen-18 (18)O as a physiological tracer is presented. The experimental aspects of the method are based on the instantaneous pyrolysis of tissue samples and subsequent conversion of the sample oxygen to carbon monoxide then to carbon dioxide whose isotopic composition is determined by isotope-ratio mass spectrometry. A mathematical procedure is then used to correct the isotopic data for interferences from the blank and memory effects and from the iodine pentoxide oxidation of CO to CO2. Laboratory animals were exposed to (18)O3 (1 ppm, 1 hr) then tissues were dried and processed for (18)O measurement. Enrichments in (18)O over natural abundance (18)O was observed in lung homogenates, nasal cavities, trachea, and pulmonary lavage fluids but not in blood of mice, rats, and rabbits. Thus, the (18)O tracing method appears to be sensitive enough to detect the reaction products of (18)O in animals exposed to near environmental concentrations of this gas.