A role for BCL-xL in regulating neuronal activity is suggested by its dramatic effects on synaptic function and mitochondrial channel activity. When recombinant BCL-xL is injected into the giant presynaptic terminal of squid stellate ganglion or applied directly to mitochondrial outer membranes within the living terminal, it potentiates synaptic transmission acutely, and it produces mitochondrial channel activity. The squid, however, is a genetically intractable model, making it difficult to apply genetic tools in squid to explore the role of endogenous BCL-xL in synaptic function. Therefore, the small molecule inhibitor ABT-737, a mimetic of the BH3-only protein BAD, binding to the BH3-binding domain pocket, was tested in squid, revealing a dual role for BCL-xL. ABT-737 slowed recovery of synaptic responses after repetitive synaptic activity, indicating that endogenous BCL-xL is necessary for timely recovery of rapidly firing synapses. Unexpectedly, however, ABT-737 also protected neurons from hypoxia-induced synaptic rundown and from increased permeability of the mitochondrial outer membrane during hypoxia. This implies that endogenous BCL-xL or a modified pro-apoptotic form of BCL-xL, N BCL-xL, contributes to injurious responses of the hypoxic synapse. To determine if ABT-737 is also an inhibitor of N BCL-xL, recombinant N BCL-xL protein was injected into the synapse or applied to mitochondrial outer membranes. ABT-737 potently inhibited both synaptic rundown and mitochondrial outer membrane permeability induced by recombinant N BCL-xL. These observations support the possibility that endogenous proteolysis or a functionally equivalent modification of BCL-xL is responsible for the deleterious effects of hypoxia on synaptic activity.