Previous studies have shown that ketone bodies (KB) exert antioxidant effects in experimental models of neurological disease. In the present study, we explored the effects of the KB acetoacetate (ACA) and beta-hydroxybutyrate (BHB) on impairment of hippocampal long-term potentiation (LTP) in rats by hydrogen peroxide (H(2)O(2)) using electrophysiological, fluorescence imaging, and enzyme assay techniques. We found that: 1) a combination of ACA and BHB (1 mM each) prevented impairment of LTP by H(2)O(2) (200 microM); 2) KB significantly lowered intracellular levels of reactive oxygen species (ROS)--measured with the fluorescent indicator carboxy-H(2)DCFDA (carboxy-2',7'-dichlorodihydrofluorescein diacetate)--in CA1 pyramidal neurons exposed to H(2)O(2); 3) the effect of KB on LTP was replicated by the protein phosphatase 2A (PP2A) inhibitor fostriecin; 4) KB prevented impairment of LTP by the PP2A activator C(6) ceramide; 5) fostriecin did not prevent the increase in ROS levels in CA1 pyramidal neurons exposed to H(2)O(2), and C(6) ceramide did not increase ROS levels; 6) PP2A activity was enhanced by both H(2)O(2) and rotenone (a mitochondrial complex I inhibitor that increases endogenous superoxide production); and 7) KB inhibited PP2A activity in protein extracts from brain tissue treated with either H(2)O(2) or ceramide. We propose that oxidative impairment of hippocampal LTP is associated with PP2A activation, and that KB prevent this impairment in part by inducing PP2A inhibition through an antioxidant mechanism.