Lipid peroxidation, free radical contents, antioxidant activity, guanididno compound metabolism and changes in amino acid contents in rat brain were studied to clarify the effect of a clinically relevant dose of hyperbaric oxygen (HBO). Seven-week-old rats were exposed to HBO at 3 atmosphere absolute for 2 hours. An increase in thiobarbituric acid reactive substances and carbon centered radicals was observed after exposure. Supreoxide dismutase-like activity also increased. The measurement of guanidino compounds revealed a significant rise in arginine and guanidinoacetic acid (GAA) levels. Since arginase activity decreased while there was no significant alteration in arginine : glycine-amidinotransferase activity, these changes in enzyme activities seem to cause the increase of arginine and GAA levels. In relation to guanidino compound metabolism, the amino acid contents were determined. An increase in glutamine and taurine levels were observed and these seemed to favor the defense mechanism. These data suggested that ongoing lipid peroxidation in the brain was caused by HBO, and that the defense mechanism against the oxidative stress was activated in response.