Many central neurons possess large acid-activated currents, yet their molecular identity is unknown. We found that eliminating the acid sensing ion channel (ASIC) abolished H+-gated currents in hippocampal neurons. Neuronal H+-gated currents and transient acidification are proposed to play a role in synaptic transmission. Investigating this possibility, we found ASIC in hippocampus, in synaptosomes, and in dendrites localized at synapses. Moreover, loss of ASIC impaired hippocampal long-term potentiation. ASIC null mice had reduced excitatory postsynaptic potentials and NMDA receptor activation during high-frequency stimulation. Consistent with these findings, null mice displayed defective spatial learning and eyeblink conditioning. These results identify ASIC as a key component of acid-activated currents and implicate these currents in processes underlying synaptic plasticity, learning, and memory.