To investigate the effect of hyperbaric oxygen(HBO)therapy on mitochondrial free radicals after transient focal cerebral ischemia in rats. The male SD rats were randomly assigned into two groups, control and HBO groups. All animals were subjected to 90 min intra-luminal middle cerebral artery occlusion (MCAO) with the regional cerebral blood flow monitored in vivo by laser Doppler flowmetry. HBO treatment was performed in a pressure chamber with 100% O(2)(3 ATM 1 h) 3 h after ischemia. Twenty-four hours after ischemia, mitochondria in the ischemic core and penumbra were isolated and the contents of H(2)O(2), O(2)(*-), MDA, SOD, GSH-PX and GSH in mitochondria were measured respectively. After cerebral ischemia-reperfusion, contents of mitochondrial H(2)O(2), O(2)(*-), MDA increased, while the SOD, GSH-PX and GSH in the mitochondria decreased significantly both in the ischemic core and the ischemic penumbra, compared with those in the normal controls(P<0.05). In the ischemic penumbra, HBO therapy increased significantly the content of O(2)(*-)(P<0.05), enhanced the activity of SOD, and decreased the level of MDA (P<0.05). However, HBO therapy did not change the level of MDA, though it also increased the content of O(2)(*-) and the activity of SOD in the ischemic core. HBO therapy had no significant effect on the contents of H(2)O(2), GSH-PX and GSH in the ischemic mitochondria. HBO therapy initiated early after acute transient cerebral ischemia in rats can increase the mitochondrial free radicals level, but also increase the activity of the anti-radical enzymes. HBO treatment inhibits the lipid peroxidation damage of mitochondria in the ischemic penumbra, but not in the ischemic core, which indicates that the mitochondrial function plays a role in the reaction of the free radical in the ischemic area after HBO therapy. Lou, Wang, Qian, Wen, Ding, , , , (2008). [Effect of hyperbaric oxygen treatment on mitochondrial free radicals after transient focal cerebral ischemia in rats]. Zhejiang da xue xue bao. Yi xue ban = Journal of Zhejiang University. Medical sciences, 2008 Sep;37(5):437-43. https://www.ncbi.nlm.nih.gov/pubmed/18925708