Several isoforms of the NMDA receptor 1 (NR1) subunits of the ionotropic NMDA (N-methyl-D-aspartate) glutamate receptor contain a consensus site for phosphorylation by protein kinase C (PKC) which, once phosphorylated results in an increased conductance through the receptor channel. Using in situ hybridization, we investigated the expression of NR1 subunits sensitive or insensitive to modification by PKC in the granule cells of the dentate gyrus of the hippocampus, following the induction of long-term potentiation (LTP). A selective 50% increase only in the levels of mRNA NR1 subunits containing this consensus sequence for PKC phosphorylation was seen 48 h after LTP induction. The change in the expression of PKC-sensitive NR1 subunits may be the molecular basis for the increased response of the post-synaptic cell to released glutamate during the maintenance phase of LTP.