Recombinant NR1a/NR2A receptors have at least a twofold higher peak open probability (peak Po) than NR1a/NR2B receptors (Chen et al., 1999; Erreger et al., 2005). This difference should affect the temporal dynamics of NMDAR signaling during synaptic transmission and plasticity. Here, we investigated the peak po of NMDARs in hippocampal synapses using the open channel blocker MK-801 in P14 and P28 mice. At these ages, the release probability is stable, as indicated by the comparable EPSC coefficient of variation. NMDA EPSCs were evoked in CA1 neurons at -40 mV by Schaffer collateral stimulation in presence of bicuculline and NBQX. After achieving a stable baseline, the stimulation was stopped and 40 mM MK-801 was perfused for 10 min. When the stimulation was resumed in presence of MK-801, NMDA EPSCs progressively decreased in amplitude, slower at P28 than at P14. To test whether the lower peak Po of synaptic NMDARs at P28 is caused by the developmental increase of the NR2A/NR2B ratio, we examined the peak Po of different NMDAR subtypes using either the NR2A-preferring antagonist NVP-AAM077 (NVP, 50 nM) or the NR2B-specific antagonist CP-101,606 (CP, 10 mM). The peak Po of the CP- and NVP-insensitive NMDAR populations were not different at P14 but were lower for the NVP- compared with the CP-insensitive NMDAR population at P28. Thus, the peak Po of synaptic NMDARs is developmentally regulated.