Pharmacologically distinct NMDA-glutamate receptor subtypes have been reported in brain. We have studied the developmental expression of the mRNA encoding the NMDA receptor NR2C subunit in the rat brain by in situ hybridization. In contrast with the adult where no hybridization signal was detected in the hippocampus, a significant signal was seen in this formation during a restricted developmental period (P7-P14). The labelling covered large parts of the hippocampus including in particular the stratum radiatum, pyramidal and oriens layers of the CA1 area. The role of the NR2C subunit in neonatal NMDA receptor properties is discussed.