Hypothalamic suprachiasmatic nucleus (SCN) is a circadian pacemaker which controls diurnal behavioral and hormonal rhythms in mammals. The SCN receives environmental light signals through the retinohypothalamic tract, and glutamate is the major excitatory neurotransmitter in the retinohypothalamic tract. In the present study, we investigated the developmental expression of the mRNAs for N-methyl-D-aspartate type glutamate receptor (NR)1, NR2A, NR2B and NR2C subunits in the rat SCN using in situ hybridization with specific riboprobes. At postnatal day 2 (P2), P8, Pl5 and P45, the high level of NRI transcripts was observed in both ventrolateral and dorsomedial subdivisions of the SCN, and the distinct expression of NR2C mRNA was principally found in the dorsomedial SCN. The weak NR2B mRNA expression was clearly found in both subdivisions of the SCN at P2 and P8, whereas specific NR2B hybridization signals were not found at Pl5 and P45. There was no specific hybridization signal of NR2A in the SCN throughout the postnatal life. These findings implicate that NR may play an important role in the neonatal SCN. In addition, this study suggests that NR1, NR2B and NR2C might be the major NR subunits in the developing SCN, whereas NRI and NR2C could be the subunit components of NR in the adult SCN.