We have analyzed genetically engineered mice in which NMDA receptor subunit 1 (NR1) is ablated only in hippocampal CA3 pyramidal cells (Nakazawa et al., 2000, SFN abstr. 564.3). In the hidden platform version of the Morris water maze task, memory acquisition and retrieval of a fixed platform position were normal in the CA3 NR knockout (mutant) mice. However, the memory retrieval was impaired when the extramaze cues were partially eliminated, suggesting that CA3 NRs are involved in spatial pattern completion. In a delayed matching-to-place (DMP) version, in which the platform position was changed day by day, the mutants showed deficits, suggesting a role for CA3 NRs in hippocampal working memory. To explore the neural mechanisms underlying these deficits in the mutants, we measured CA1 place cell activity of freely exploring animals using in vivo tetrode recording techniques. Preliminary results comparing mutants and NR1-floxed (control) mice showed that place field size and peak firing rate of CA1 place ce...