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.