The present study was attempted to assess the mechanisms underlying the beneficial effects of hyperbaric oxygen (HBO2; 100% O2 at 253 kpa) in treating experimental heatstroke. Anesthetized rats were divided into five major groups: the normothermic control (NC) rats treated with normobaric air (NBA; 21% O2 at 101 kpa) (NC+NBA), the NC rats treated with HBO2 (NC+HBO2), the heatstroke (HS) rats treated with NBA (HS+NBA), the HS rats treated with hyperbaric air (HBA; 21% at 253 kpa) (HS+HBA), and the HS rats treated with HBO2 (HS+HBO2). The HS groups were exposed to heat (43o) for exactly 68 min and then allowed to recover at 26oC. HBA or HBO2 was adopted 68 min or 78 min after the start of heat exposure. The survival time values for (HS+NBA) rats, (HS+HBA) rats at 68 min, (HS+HBA) rats at 78 min, (HS+HBO2) rats at 68 min, and (HS+HBO2) rats at 78 min were found to be 90±3 mins, 133±12 mins, 109±9 mins, 240±18 mins, and 170±15 mins, respectively. Resuscitation with HBA or HBO2 at 68 mins was superior to those treated at 78 mins in prolonging the survival time values. All (HS+NBA) animals displayed hyperthermia, hypotension, and increased cellular levels of ischemia, oxidative stress and damage markers, pro-inflammatory cytokines and an indicator of polymorphonuclear cells accumulation in their hypothalamus as compared to those of normothermic controls. The heat-induced hyperthermia was not affected by HBA or HBO2 treatment. However, heat-induced hypotension and hypothalamic ischemia, oxidative stress, neuronal damage, and inflammation were all significantly reduced by HBA or HBO2 therapy. Compared to those of HBA therapy, HBO2 therapy had significantly higher beneficial effect in treating heatstroke. Our results suggested that HBO2 improved heatstroke outcomes in part by restoring normal hypothalamic function. Delaying the onset of HBO2 therapy reduced the therapeutic efficiency.