While studies showed that aging is accompanied by increased exposure of the brain to oxidative stress, others have not detected any age-correlated differences in levels of markers of oxidative stress. Use of conventional markers of oxidative damage in vivo, which may be formed ex vivo and/or eliminated by endogenous metabolism, may explain these conflicting results. Recently, F(2)-isoprostanes and F(4)-neuroprostanes, peroxidation products of arachidonic acid and docosahexaenoic acid, respectively, have been identified as sensitive and reliable markers of oxidative injury. Therefore, this study was designed to quantify brain levels of F(2)-isoprostanes and F(4)-neuroprostanes and their precursors in 4, 10, 50, and 100 week old male Fischer 344 rats. Data show that levels of F(2)-isoprostanes and F(4)-neuroprostanes were comparable in all animal age groups. However, levels of F(4)-neuroprostanes were approximately 20-fold higher than those of F(2)-isoprostanes in all age groups, despite the fact that brain levels of docosahexaenoic acid were only twice as high as those of arachidonic acid. Based on our findings, it is concluded that aging is not accompanied by enhanced brain susceptibility to oxidative stress. Furthermore, the metabolically active gray matter of the brain, where docosahexaenoic acid is abundant, appears more susceptible to oxidative stress than the white matter.