P>Highly activated microglia and followed excessive expression of inflammatory cytokines are associated with neuroexcitotoxic injuries. We use electrophysiological techniques, ELISA, western-blot, RT-PCR assay and TUNEL method to explore whether over-produced tumor necrosis factor alpha (TNF alpha) released from activated microglia results in neuronal injuries, and further causes apoptosis through increasing excitotoxicity of hippocampal neurons. Our data showed that kainic acid (KA) activated microglia highly expressed TNF alpha, mRNA and protein. KA activated microglia conditioned media ((KA-MCM) significantly enhanced the amplitude of the population spike at rat's hippocampal CA3 region. It also increased the Ca2+ current amplitude and density in cultured hippocampal neurons, as well as the high expression of NMDAR1, iNOS, and caspase 3 mRNA and protein at both hippocampal neurons and tissues. KA-MCM also increased TUNEL-positive cells in hippocampal neurons, whereas addition of anti-TNF alpha to the KA-MCM before its application significantly reduced those effects. These studies suggest that TNF alpha derived from KA activated microglia increases excitotoxicity of hippocampal neurons, and might induce neuronal apoptosis in vitro and in vivo.