The subunit composition of N-methyl-D-aspartate (NMDA) receptors affects their function under normal and pathological conditions. Functional NMDA receptors are expressed in lower motor neurons, but their subunit composition has not been defined. Here, we employed electrophysiology, quantitative PCR, and immunohistochemistry to investigate the subunit composition of NMDA receptors in postnatal motor neurons of the Wistar rat facial nucleus (FN). Whole-cell patch clamp recordings of acutely dissociated motor neurons from postnatal days 3 and 4 (P3-P4) showed that ifenprodil, a specific antagonist of the NMDA receptor 2B (NR2B) subunit, inhibited 91.62%+/-2.02% of NMDA-induced current, whereas NVP-AAM007, a specific antagonist of the NMDA receptor 2A (NR2A) subunit, inhibited much less of the current (16.69%+/-3.28%). Starting from P5, the inhibitory effects of ifenprodil and NVP-AAM007 gradually decreased and increased, respectively, such that the effect of NVP-AAM007 exceeded that of ifenprodil by P10. At P14, most of the NMDA-induced current was inhibited by NVP-AAM007 (84.59%+/-3.35%). Consistent with this, NR2B mRNA and protein were expressed highly at P3 and then gradually decreased by more than 75% by P14 in FN motor neurons, while NR1 was expressed stably over the same ages. However, NR2A mRNA and protein showed relatively constant levels between P3-P10 and decreased to 45% and 75% of the P3 level, respectively, by P14. Thus, analysis of functional NMDA receptors is critical to revealing subunit switching, which may be an important step in postnatal development of FN motor neurons.