Lipid peroxidation was measured in brains and lungs from mice which had been exposed to 100% O2 at 445 kPa, 515 kPa and 585 kPa for 30 min. These treatments produced varying degrees of convulsive activity and negligible to moderate lung damage. Lipid peroxidation was estimated by the thiobarbituric acid method and by analysis of Schiff bases. Lipid peroxidation did not increase in either lungs or brains after hyperbaric oxygen exposure, nor was there any difference between tissues from control or hyperoxic mice in the degree of lipid peroxidation which occurred during in vitro incubation of homogenates at 37 degrees in air for 150 min. Similarly, no increased lipid peroxidation was seen in lung tissue from mice exposed to normobaric 100% O2 for either 60 or 72 hr.