To examine whether reactive oxygen species (ROS) production is affected by arterial oxygen content (CaO 2 ) in attempted resuscitation to restore blood pressure from hemorrhagic shock (HS) or not.Under light anesthesia and spontaneous beating, 16 rats underwent HS for 80min, during which 3.0mL/100g of blood was withdrawn, followed by resuscitation attempt for 70min. At 80min, rats were randomized into a high-CaO 2 group (Group 1, transfusion under fractional inspired oxygen (F I O 2 ) of 1.0, n=8) or a low-CaO 2 group (Group 2, fluid administration under F I O 2 of 0.21, n=8). In each group, either blood or lactate Ringer's (LR) solution was infused to maintain mean arterial pressure ≥75mmHg under each F I O 2 concentration. CaO 2 , O 2 utilization coefficient (UC) and plasma %CoQ9 were compared between groups.Mean infused volume for attempted resuscitation was 7.6±1.0mL of blood in Group 1, and 31.4±5.5mL of LR solution in Group 2. At the end of resuscitation, CaO 2 was 18.5±1.2 vol% in Group 1, almost double the 9.1±0.8 vol% in Group 2 (P<0.01). O 2 UC and %CoQ9 in all rats increased from baselines of 0.25±0.12 and 7.6±1.8% to 0.44±0.13 and 9.7±1.8% after resuscitation, respectively (P<0.05 vs. baseline for each), but did not differ significantly between the groups.In a rat HS model, attempted resuscitation to restore blood pressure increased O 2 UC as well as %CoQ9. However, the magnitude of %CoQ9 increase that represents ROS production is not affected by CaO 2 during resuscitation from HS.