1. Postnatal development of synaptic efficacy was studied at a single glutamatergic synapse formed by the calyx of Held in the rat brainstem. Throughout postnatal development from day 7 (P7) to day 14 (P14), both the excitatory postsynaptic currents (EPSCs) evoked at a basal frequency (0.05 Hz) and spontaneous miniature EPSCs (mEPSCs) remained similar in their mean amplitudes, but became faster in their decay times. 2. During repetitive stimulation at 1-100 Hz, EPSCs underwent a depression. The magnitude of the depression significantly decreased from P7 to P14, whereas the time course of recovery from depression (after 10 Hz stimulation) remained similar throughout development. 3. The size of the readily releasable pool (RRP) of synaptic vesicles (N) and the release probability (p) were estimated from the cumulative amplitude histogram of EPSCs during high frequency stimulation. From P7 to P14, N increased 2-fold, whereas p decreased to a similar extent. 4. The open channel blocker MK-801 caused an activity-dependent attenuation of NMDA receptor-mediated EPSCs. The blocking rate became slower from P7 to P14, further supporting the developmental decrease in p. 5. Given that the mean amplitudes of mEPSCs (q) and evoked EPSCs (Npq) remain constant throughout the developmental period, these results suggest that a developmental increase in N compensates for a concomitant decrease in p. We conclude that the developmental decrease in the release probability will establish a stable synapse at which only a small fraction of releasable synaptic vesicles is depleted during high frequency transmission.