Objective: Preliminary evidence suggests local brain tissue oxygenation (PbtO2) values ≤15 mm Hg following severe traumatic brain injury (TBI) represent brain tissue hypoxia. Accordingly, many neurotrauma units attempt to maintain PbtO2 ≥ 20 mm Hg to avoid hypoxia. This study tested the impact of a short (2hr) trial of normobaric hyperoxia on measures of oxidative stress. We hypothesized this treatment would positively affect cerebral oxygenation but negatively affect the cellular environment via oxidative stress mechanisms. Methods: Cerebrospinal fluid (CSF) was serially assessed in 11 adults (9 male, 2 female), aged 26+/-1.8 yrs with severe TBI (Glasgow Coma Scale score 6+/-1.4) before, during, and after a FiO2 = 1.0 challenge for markers of oxidative stress including lipid peroxidation (F2-isoprostane [ELISA]), protein oxidation (protein sulfhydryl [fluorescence]) and antioxidant defenses (total antioxidant reserve (AOR) [chemiluminescence] and glutathione [fluorescence]). Physiological parameters, [PbtO2, arterial oxygen content (PaO2), intracranial pressure (ICP), mean arterial pressure (MAP), and cerebral perfusion pressure (CPP)] were assessed at the same time points. Results: Mean (+/-SD) PbtO2 and PaO2 levels significantly changed for each time point, [before 27.3+/-7.4 mm Hg, 173.1+/-51.4 mm Hg; during 93.9+/-58.1, 385.5+/-108.3; and after the FiO2 challenge 29.3+/-13.0, 171.8+/-45.1, (p=.04; .01), respectively]. Oxidative stress markers, antioxidant reserve defenses and ICP, MAP and CPP did not significantly change for any time period. Conclusion: These preliminary findings suggest that brief periods of normobaric hyperoxia do not produce oxidative stress and/or change antioxidant reserves in CSF. Additional studies are required to examine extended periods of normobaric hyperoxia in a larger sample.