1. Whole-cell patch-clamp techniques were used to record pharmacologically isolated NMDA receptor-mediated EPSCs (NMDA EPSCs) from CA1 pyramidal cells (PCs) in hippocampal slices from 4-day-old to 36-week-old mice, in order to characterize developmental changes in functional properties and subunit composition of synaptic NMDA receptors. 2. During the first postnatal weeks the dendritic tree of CA1 PCs stained with biocytin increased both in size and in complexity. This was associated with an increase in amplitude of the focally evoked NMDA EPSCs recorded either in nominally Mg(2+)-free or Mg(2+)-containing saline. In adult PCs (> 5 weeks old) EPSC amplitude was 4-fold larger than in very young (up to 2 weeks old) neurones. 3. The sensitivity of NMDA EPSCs to blockade by Mg2+ did not change with age. In very young, intermediate and adult PCs the EPSC-voltage relation displayed an area of negative slope conductance at membrane potentials more negative than -30 mV. The apparent Kd values of the NMDA receptors for Mg2+ at 0 mV were 7.8 +/- 6.4, 10.4 +/- 14.1 and 6.5 +/- 4.7 mM in very young, intermediate and adult neurones, respectively. 4. The decay of the NMDA EPSC in both young and adult neurones could be described by the sum of a fast and a slow exponential function. Both EPSC rise time and fast and slow decay time constants measured at -60 mV, decreased with age. 5. The decay of NMDA EPSCs in young versus adult PCs was differentially modulated by membrane voltage. In young PCs depolarization slowed both the fast and the slow EPSC components. In adult PCs depolarization slightly accelerated the initial EPSC decay, though the overall duration of the EPSC did not change. The rise time of the EPSCs was not affected by voltage at any age. 6. The subunit-selective NMDA receptor antagonist ifenprodil similarly blocked iontophoretic NMDA-induced currents and NMDA EPSCs. In both young and adult PCs, the concentration-response curves for this effect disclosed distinct low and high affinity binding sites for ifenprodil. 7. In young PCs, low and high affinity binding sites for ifenprodil were about equally expressed (57 versus 43%, respectively), whereas in adult PCs, synaptic NMDA receptors expressed a majority (78%) of low affinity binding sites for ifenprodil. 8. The long duration of NMDA EPSCs (and by implication, of Ca2+ transfer through NMDA receptor channels) and its further prolongation by depolarization in young PCs are consistent with heightened NMDA-dependent neuronal plasticity early in development. The age-related changes in these properties may result from a developmental change in NMDA receptor subunit composition.