BACKGROUND It is postulated that application of hyperbaric oxygenation may induce the production of radicals after HBO. Higher oxygenation and transport of oxygen increase the mitochondrial energy turnover, whereas inner mitochondrial radical formation decreases. METHODS Several markers of oxidative stress in healthy volunteers (n = 21), including plasma carbonyl proteins (CP), malondialdehyde (MDA), oxidized LDL (oxLDL), 8-hydroxy-deoxyguanosine (8-OHdG), and erythrocyte glutathione peroxidase (GPx) activity are measured before, during, and after HBO. RESULTS Median plasma concentrations of CP decreased significantly during HBO compared to CP levels before HBO (from 77.1 to 61.7 pmol/mg; p < 0.001) and increased again after HBO (to 78.1 pmol/mg; p = 0.035). 8-OHdG decreased significantly during HBO (8.1 ng/mL; p < 0.001) and remained constant after HBO (8.1 ng/mL) compared to "before HBO" (9.4 ng/mL). MDA increased significantly from 0.92 μM (before HBO) to 1.26 μM (during HBO, p < 0.01) and decreased again to 1.00 μM (after HBO, p = 0.023). Erythrocyte GPx activity also increased significantly during HBO (26.5 ± 14.7; p = 0.005), but not after HBO (25.6 ± 17.2 IU/mg). A negative correlation was observed between GPx and MDA only during HBO (r = -0.518; p = 0.016). CONCLUSIONS We assume that higher oxygen consumption decreases, on the one hand, the inner mitochondrial generation of free radicals and, on the other, RONS by activation of detoxifying enzymes like GPx.