We investigated the effects of interleukin-4 (IL-4), a well-known anti-inflammatory cytokine, on thrombintreated rat hippocampi in vivo. Intrahippocampal injection of thrombin resulted in a significant loss of hippocampal CA1 neurons, as determined by Nissl staining and NeuN immunohistochemistry. Thrombin-induced neurotoxicity was accompanied by substantial microglial activation, as demonstrated by OX-42 immunohistochemistry. In parallel, Western blot analysis and hydroethidine histochemistry revealed activation of NADPH oxidase (as demonstrated by increased translocation of the cytosolic proteins p67 and p47 ), generation of reactive oxygen species (ROS), and oxidative damage in the hippocampal CA1 area, where degeneration of hippocampal neurons was evident. Interestingly, immunohistochemical and biochemical analysis demonstrated that intrahippocampal injection of thrombin increased immunoreactivity and levels of IL-4 as early as 8 h post-treatment, reaching a peak at 7 days that was maintained for up to 14 days. Moreover, double-label immunohistochemistry detected IL-4 immunoreactivity solely in activated microglia. In experiments to explore the involvement of IL-4 in neurotoxicity, IL-4-neutralizing antibodies significantly increased the survival of CA1 hippocampal neurons at 7 days post-thrombin treatment. Consistent with these results, IL-4 neutralization inhibited activation of NADPH oxidase, ROS production and oxidative damage. Thus, the present study is the first to demonstrate that IL-4 generates microglial NADPH oxidase-derived oxidative stress and leads to the degeneration of hippocampal neurons in vivo, as occurs in Alzheimers disease.