The present study was attempted to evaluate the therapeutic effects of activated protein C and/or hyperbaric oxygen in an animal model of heatstroke. Sixty-eight minutes heat stress (43 degrees C) initiated, the anesthetized rats were randomized to several groups and administered: 1) no resuscitation (vehicle solution plus normabaric air, 2) intravenous activated protein C (1mg in 1ml of normal saline per kg of body weight), 3) hyperbaric oxygen (100% oxygen at 202kpa for 17min), and 4) intravenous activated protein C plus hyperbaric oxygen. Another group of rats exposed to room temperature (26 degrees C) was used as normothermic controls. Blood sampling was 0min, 70min, and 85min after heat stress initiated. When the vehicle-treated rats underwent heat exposure, their survival time values found were to be 19-25min. Resuscitation with activated protein C or hyperbaric oxygen significantly and equally improved survival during heatstroke (134-159min). As compared with those of activated protein C or hyperbaric oxygen alone, combined activated protein C and hyperbaric oxygen significantly had higher survival time values (277-347min). All vehicle-treated heatstroke animals displayed systemic response, hypercoagulable state, and hepatic and renal dysfunction. Combined activated protein C and hyperbaric oxygen therapy reduced these heatstroke reactions better than activated protein C or hyperbaric oxygen alone. The results indicate consequently, combined activated protein C and hyperbaric oxygen therapy heightens benefit in combating heatstroke reactions.