Hippocampal pyramidal cells are called place cells because each cell tends to fire only when the animal is in a particular part of the environment-the cell's firing field Acute pharmacological blockade of N- methyl-D-aspartate (NMDA) glutamate receptors was used to investigate how NMDA-based synaptic plasticity participates in the formation and maintenance of the firing fields. The results suggest that the formation and short-term stability of firing fields in a new environment involve plasticity that is independent of NMDA receptor activation. By contrast, the long-term stabilization of newly established firing fields required normal NMDA receptor function and, therefore, may be related to other NMDA-dependent processes such as long-term potentiation and spatial learning.