The effect of hyperbaric oxygen (HBO) on cyclooxygenase-2 (COX-2) expression after transient focal ischemia was evaluated. A rat middle cerebral artery occlusion/reperfusion (MCAO) model was produced using the intraluminal filament method. After 2 h of occlusion, 24 h of reperfusion, brains were removed. Three atmospheres absolute HBO for 1 h was administered at 6 h after reperfusion. The infarct volume was evaluated by 2,3,7-triphenyltetrazolium chloride staining. COX-2 mRNA expression was measured by reverse transcription polymerase chain reaction, and COX-2 protein expression was analyzed by Western blot. The results showed that HBO applied at 6 h after reperfusion significantly reduces infarct area as compared with no-treatment group. HBO decreased COX-2 mRNA and protein levels, which were upregulated after ischemia/reperfusion. HBO had no direct effect on COX-2 protein expression in matched normal rats. We conclude that (1) early intervention with HBO within 6 h reduces infarction. (2) The neuroprotective effect of HBO might lead to an inhibition of COX-2 over-expression in cerebral cortex.