The effectiveness of chlorogenic acid and its main metabolites, caffeic and quinic acids, against oxidative stress was investigated. Resveratrol, another natural phenolic compound, was also tested for comparison. Rat cortical slices were incubated with 200 mu M H2O2 for 1 h, and alterations in oxidative stress parameters, such as 2, 3, 5-triphenyltetrazolium chloride (TTC) staining and the production of both malondialdehyde (MDA) and reactive oxygen species (ROS), were assayed in the absence or presence of phenolic compounds. Additionally, the effectiveness of chlorogenic acid and other compounds on H2O2-induced increases in fluorescence intensities were also compared in slice-free incubation medium. Although quinic acid failed, chlorogenic and caffeic acids significantly ameliorated the H2O2-induced decline in TTC staining intensities. Although resveratrol also caused an increase in staining intensity, its effect was not dose-dependent; the high concentrations of resveratrol tested in the present study (10 and 100 mu M) further lessened the staining of the slices. Additionally, all phenolic compounds significantly attenuated the H2O2-induced increases in MDA and ROS levels in cortical slices. When the IC50 values were compared to H2O2-induced alterations, chlorogenic acid was more potent than either its metabolites or resveratrol for all parameters studied under these experimental conditions. In slice-free experimental conditions, on the other hand, chlorogenic and caffeic acids significantly attenuated the fluorescence emission enhanced by H2O2 with a similar order of potency to that obtained in slice-containing physiological medium. These results indicate that chlorogenic acid is a more potent phenolic compound than resveratrol and its main metabolites caffeic and quinic acids against H2O2-induced alterations in oxidative stress parameters in rat cortical slices.