Abstract: NMDA receptors represent a subtype of the ionotropicglutamate receptor family, comprising three classes of subunits (NR1, NR2A-D,NR3), which exhibit distinct patterns of regional and developmental expressionin the CNS. Recently, some NMDA receptor subunits have also been described inadult extraneuronal tissues and keratinocytes. However, their developmentalexpression patterns are currently unknown. With use of RT-PCR and western blotanalysis, the expression of NMDA receptor subunit NR2B was investigated in thedeveloping rat heart. NR2B mRNA and protein were detected in heart tissue ofrats from embryonic day 14 until postnatal day 21 but disappeared 10 weeksafter birth. In contrast, no NMDA receptor subunit NR1,α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor subunitGluR2, or anchoring postsynaptic density protein-95 could be detected in ratheart at any developmental stage. Confocal microscopy of cultured cardiacmyocytes (CMs) from neonatal rats revealed distinct NR2B staining mainly ofintracellular structures. However, no functional NMDA receptor could bedetected on CMs by whole-cell recordings. In conclusion, high concentrationsof NR2B protein can be detected in early rat heart development, but itsfunction still remains elusive.