To investigate the role of synaptic plasticity in the place-specific firing of the hippocampus, we have applied multiple electrode recording techniques to freely behaving mice with a CA1 pyramidal cell–specific knockout of the NMDAR1 gene. We have discovered that although the CA1 pyramidal cells of these mice retain place-related activity, there is a significant decrease in the spatial specificity of individual place fields. We have also found a striking deficit in the coordinated firing of pairs of neurons tuned to similar spatial locations. Pairs have uncorrelated firing even if their fields overlap. These results demonstrate that NMDA receptor–mediated synaptic plasticity is necessary for the proper representation of space in the CA1 region of the hippocampus.