Hyperoxia has been considered a model of free radical reactive oxygen species production in aging and age-related disorders. Previously, we studied the membrane protein alterations that occur during hyperoxia; we found that exposure of young animals to 24 h of hyperoxia provided the greatest degree of oxidation of cortical synaptosomal membrane proteins. We reasoned that free radical oxidation was involved in this protein oxidation. In accordance, in the current study we investigated the protective nature of two known free radical scavengers, N-tert-butyl-alpha-phenylnitrone (PBN) and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (Tempol), against 24-h hyperoxia damage. The three techniques used in this study were electron paramagnetic resonance (EPR) protein-specific spin labeling, assay of the activity of the oxidatively sensitive enzyme glutamine synthetase (GS), and measurement of protein carbonyl content. Before hyperoxia, gerbils received intraperitoneal injections of varying concentrations of either of the two free radical scavengers. After 30 min, the gerbils were exposed to 90-100% O2 for 24 h. For the spin labeling experiments, cortical synaptosomes were isolated from gerbils. The membrane proteins were spin labeled with the thiol-specific label MAL-6 (2,2,6,6-tetramethyl-4-maleimidopiperidin-1-oxyl). As in our earlier study, the EPR spectral parameter of MAL-6-labeled membranes, the W/S ratio, decreased with hyperoxia (p < 0.00001). This effect was lessened significantly with administration of PBN (p < 0.0003) or Tempol (p < 0.00003). For the GS and protein carbonyl assays, cortical proteins were used. The activity of the GS decreased with hyperoxia (p < 0.000005), and this effect likewise was lessened with administration of PBN (p < 0.004) or Tempol (p < 0.002). The protein carbonyl content increased with hyperoxia (p < 0.0002), and there was a protective effect found with Tempol (p < 0.000001). The optimum doses for PBN and Tempol were 20 and 5 mg/kg, respectively. The results are discussed with reference to the use of free radical scavengers as potential antiaging agents.