Long-term storage of information is a hallmark feature of the brain, yet routine turnover of synaptic receptors appears to be intrinsically paradoxical to this capability. To investigate how the brain preserves its delicate synaptic efficacies, we generated inducible and reversible knockout mice in which the NMDA receptor can be temporarily switched off in the forebrain specifically during the storage stage. Retention of 9-month contextual and cued fear memories is severely disrupted by prolonged, but not transient, loss of the NMDA receptor that occurs 6 months after initial training and at least 2 months prior to memory retrieval. Normal learning and memory function in subsequent tasks following the 9-month retention tests suggest that the observed retention deficits did not result from recall or performance impairment. Thus, our study reveals a hitherto unrecognized role of the NMDA receptor in dynamically maintaining the long-term synaptic stability of memory storage circuits in the brain.