We have studied spatial memory in genetically engineered mice with CA3 pyramidal cell ablation of the NMDA receptor subunit 1 (CA3-NR1 KO mice) (Nakazawa et al., 2000, SFN abstr. 564.3). We previously discovered that these mice were unable to remember novel locations of a hidden platform in a delayed matching-to-place version of water maze task. This deficit disappeared when familiar locations were used, suggesting that the animals relied on long-term memory to solve this short-term memory task (Nakazawa et al., 2001, SFN abstr. 744.2). Here we show, by in vivo CA1 place cell recording in familiar and novel linear tracks, that CA1 place cell activities of CA3-NR1 KO mice were enhanced and that their place fields were less confined when the mice explored a novel track. The CA1 spatial representation was restored when these mice explored the same track the following day. The CA1 place cell activities were also enhanced while the mice explored a novel open field. Once again, the activities were not enhanced t...