1. In vitro slices of frontal neocortex were prepared from rat pups at various ages after birth: postnatal days (PN) 3-5, 6-8, and 9-14. Using whole-cell patch-clamp techniques, both spontaneous and evoked excitatory postsynaptic currents (EPSCs) were recorded from voltage-clamped layer II- III pyramidal neurons. Developmental changes in EPSCs were examined. 2. Four properties of miniature EPSCs (mEPSCs) were studied: rise time, amplitude, decay time constant (τ), and frequency. These parameters were not tetrodotoxin sensitive and did not exhibit significant developmental changes during the first two postnatal weeks. 3. mEPSCs occurred approximately every 2-3 s and had peak amplitudes of 25-30 pA. Within each age group, certain parameters of mEPSCs were voltage dependent. mEPSC rise time and decay τ were significantly increased at depolarized potentials (-30 to -45 mV) relative to hyperpolarized (-75 to -90 mV) or resting membrane potential (RMP) (-60 to -70 mV). 4. At threshold stimulation intensity, EPSCs were evoked in an 'all-or-none' manner. The amplitude and decay τ of evoked unitary EPSCs and mEPSCs were not significantly different. As stimulation intensity was increased, a late EPSC component appeared that was not seen in mEPSCs. At suprathreshold stimulus intensities, EPSC duration was significantly longer in PN 3-5 than in PN 9-14 neurons. 5. The N-methyl-D- aspartate (NMDA) receptor antagonist D(-)2-amino-5-phosphonovaleric acid (APV, 10 μM) significantly decreased mEPSC decay τ and frequency only at depolarized membrane potentials. Likewise, EPSCs were depressed by APV to a greater extent at depolarized potentials, and the depression was mainly of the late component. mEPSCs recorded at RMP were blocked by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (5 μM). 6. Removal of extracellular Mg2+ reversibly increased the decay τ of mEPSCs at RMP but not at depolarized membrane potentials. The decay τ and duration of evoked EPSCs were also increased in zero Mg2+. These effects were reversible with application of APV. All effects of zero Mg2+ on mEPSCs and EPSCs were observed as early as PN 3-5. 7. These results indicate that the basic kinetic properties of mEPSCs are present by PN 3-5 and do not change significantly over the first two postnatal weeks. NMDA receptor activation contributes to mEPSCs and sensitivity to Mg2+ appears as early as PN 3-5. Unitary EPSCs resemble mEPSCs, but a late NMDA receptor-mediated component appears in EPSCs as stimulus intensity is increased. The developmental decrease in the late EPSC component as well as its apparent increase in voltage sensitivity cannot be attributed to changes in characteristics of miniature synaptic currents.