1. In vitro slices of frontal neocortex were prepared from rat pups 3-14 days of age. Whole-cell patch-clamp recordings were obtained from layer II- III cortical neurons, and measurements of passive membrane properties were made. The development of evoked synaptic excitation and inhibition was also examined with the use of current- and voltage-clamp techniques. 2. Pharmacological separation of excitatory synaptic activity into both N- methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated components was accomplished by application of D(-)2-amino-5-phosphonovaleric acid (APV), D(- )2-amino-7-phosphonoheptanoic acid (AP7), and 6-cyano-7-nitroquinoxaline- 2,3-dione (CNQX). Inhibitory synaptic events were described according to their reversal potentials and modulation by the GABA(A) receptor antagonist bicuculline methiodide (BMI). 3. Pups were grouped into three categories on the basis of age: postnatal day (PN) 3-5, PN 6-8, and PN 9-14. In slices from PN 3-5 pups, neurons exhibited high input resistances (R(n)) and relatively low resting membrane potentials (RMP). R(n)s decreased, and RMPs became more negative with development. At all ages studied, current-voltage relationship measured in current clamp were relatively linear, with inward rectification observed in some neurons at hyperpolarized membrane potentials. Neurons in each group were capable of firing overshooting action potentials. 4. Local stimulation in layer IV-V at 0.033 Hz elicited depolarizing excitatory postsynaptic potentials (EPSPs) in neurons from all three age groups. In PN 3-5 neurons, EPSPs were characterized by a long duration and latency to peak. By PN 6-8, EPSPs had decreased significantly in both duration and latency- to-peak. Some neurons responded with a single-component EPSP, whereas others exhibited multicomponent EPSPs consisting of distinct early and late components. In PN 3-5 neurons, increasing the frequency of stimulation from 0.033 to 1 Hz resulted in an overall decrease in the amplitude of the entire EPSP, whereas in PN 6-8 neurons the main decrease was observed in the late EPSP. 5. Excitatory postsynaptic currents (EPSCs) recorded in both PN 3-5 and PN 6-8 neurons were shorter in duration than corresponding EPSPs and consisted of both early and late components. Early EPSCs routinely increased in amplitude with hyperpolarization at all ages. In PN 3-5 neurons, the voltage dependence of late EPSCs was variable. By PN 6-8, late EPSCs always exhibited a region of reduced amplitude from -45 to -90 mV. The reversal potential for both early and late EPSCs was near +10 mV. 6. Bath application of the NMDA receptor antagonists APV or AP7 (10 μM) decreased the amplitude of late EPSPs and EPSCs without significantly affecting early components. The depression of the late EPSP and EPSC was significantly greater in PN 3-5 than in PN 6-8 neurons. In the presence of the non-NMDA receptor antagonist CNQX (5 μM), an NMDA receptor-mediated response could still be evoked. All excitatory synaptic activity was blocked in the presence of CNQX and APV. 7. Inhibitory postsynaptic potentials (IPSPs) were not detected in PN 3-5 neurons and rarely observed in PN 6-8 neurons. Early chloride-dependent IPSPs were routinely observed in PN 9-14 neurons. Late potassium-dependent IPSPs were also evident in the latter age group. Bath application of the GABA(A) receptor antagonist BMI (10 μM) had no effect on evoked responses in PN 3-5 neurons. In PN 6-8 neurons, BMI enhanced the amplitude and duration of late EPSPs. By PN 9-14, BMI induced epileptiform burst responses. 8. These results demonstrate that evoked NMDA and non-NMDA receptor-mediated synaptic currents are both present as early as PN 3. Excitatory synaptic activity in the neonatal rat neocortex is of long duration, and the NMDA component of evoked synaptic activity is larger during the first postnatal week. Evoked synaptic inhibition is mainly absent during the first postnatal week and gradually develops over the second week. The prolonged duration of NMDA-mediated components during the early postnatal period may contribute to the trophic influences of NMDA receptors in development.