Transient global ischemia in rats induces delayed death of hippocampal CA1 neurons. Early events include caspase activation, cleavage of anti-death Bcl-2 family proteins and large mitochondrial channel activity. However, whether these events have a causal role in ischemia-induced neuronal death is unclear. We found that the Bcl-2 and Bcl-xL inhibitor ABT-737, which enhances death of tumor cells, protected rats against neuronal death in a clinically relevant model of brain ischemia. Bcl-xL is prominently expressed in adult neurons and can be cleaved by caspases to generate a pro-death fragment, ΔN-Bcl-xL. We found that ABT-737 administered before or after ischemia inhibited ΔN-Bcl-xL–induced mitochondrial channel activity and neuronal death. To establish a causal role for ΔN-Bcl-xL, we generated knock-in mice expressing a caspase-resistant form of Bcl-xL. The knock-in mice exhibited markedly reduced mitochondrial channel activity and reduced vulnerability to ischemia-induced neuronal death. These findings suggest that truncated Bcl-xL could be a potentially important therapeutic target in ischemic brain injury.