The tumor suppressor phosphatase PTEN can promote apoptosis of mitotic cells by inhibiting activation of the cell survival kinase Akt. PTEN is essential for normal embryonic development, PTEN expression is associated with neuronal differentiation, and deletion of PTEN in the mouse brain results in seizures, ataxia, and other abnormalities. However, the possible roles of PTEN in regulating neuronal survival are not known. We provide evidence that PTEN sensitizes hippocampal neurons to excitotoxic death in culture and in vivo. Overexpression of wild-type PTEN decreased, while a dominant-negative PTEN increased, levels of activated Akt in cultured hippocampal neurons. Wild-type PTEN promoted, while dominant-negative PTEN prevented, apoptotic death of neurons exposed to the excitatory amino acid neurotransmitter glutamate. Hippocampal neurons of mice with reduced PTEN levels were more resistant to seizure-induced death compared to wild-type littermates. These findings demonstrate a cell death function of PTEN in hippocampal neurons and identify PTEN as a potential therapeutic target for neurodegenerative disorders that involve excitotoxicity and apoptosis. The ability of PTEN to modify neuronal sensitivity to glutamate also suggests possible roles for PTEN in regulating developmental and synaptic plasticity.