Ozone is known to induce lipid peroxidation of lung tissue, although no direct evidence of free radical formation has been reported. The authors have used the electron paramagneticresonance (EPR) spin-trapping technique to search for free radicals produced in vivo by ozone exposure. The spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) was administered i.p. to male Sprague-Dawley. The rats were then exposed for 2 hrs to either 0, 0.5, 1.0, 1.5 or 2.0 ppm ozone with 8% CO2 to increase their respiratory rate. The concentration of the radical adduct increased as a function of ozone concentration. After administration of 4-POBN, rats were exposed for either 0.5, 1.0, 2.0 or 4.0 hrs to either 0 or 2.0 ppm ozone (with CO2). The radical adduct concentration of the ozone-exposed groups at exposure times of 2.0 and 4.0 hrs was significantly different from that of the corresponding air control groups. A correlation was observed between the radical adduct concentration and the lung weight/body weight ratio. These results demonstrate that ozone induces the production of free radicals in rat lungs during inhalation exposure and that radical production may be involved in the induction of lung edema by ozone. This is the first direct evidence for ozone-induced free radical production in vivo.