Mutations in the presenilin genes are the main cause of familial Alzheimer's disease. Loss of presenilin activity and/or accumulation of amyloid-β peptides have been proposed to mediate the pathogenesis of Alzheimer's disease by impairing synaptic function. However, the precise site and nature of the synaptic dysfunction remain unknown. Here we use a genetic approach to inactivate presenilins conditionally in either presynaptic (CA3) or postsynaptic (CA1) neurons of the hippocampal Schaeffer-collateral pathway. We show that long-term potentiation induced by theta-burst stimulation is decreased after presynaptic but not postsynaptic deletion of presenilins. Moreover, we found that presynaptic but not postsynaptic inactivation of presenilins alters short-term plasticity and synaptic facilitation. The probability of evoked glutamate release, measured with the open-channel NMDA (N-methyl-D-aspartate) receptor antagonist MK-801, is reduced by presynaptic inactivation of presenilins. Notably, depletion of endoplasmic reticulum Ca2+ stores by thapsigargin, or blockade of Ca2+ release from these stores by ryanodine receptor inhibitors, mimics and occludes the effects of presynaptic presenilin inactivation. Collectively, these results indicate a selective role for presenilins in the activity-dependent regulation of neurotransmitter release and long-term potentiation induction by modulation of intracellular Ca2+ release in presynaptic terminals, and further suggest that presynaptic dysfunction might be an early pathogenic event leading to dementia and neurodegeneration in Alzheimer's disease.