1. Neocortical brain slices were prepared from animals 8-15 days of age and maintained in vitro. Intracellular recordings were obtained from neurons in cortical layers 2-3. The role of synaptic activity and excitatory amino acid receptors in generation of picrotoxin-induced ictal-like epileptiform activity in the immature neocortex was investigated. D-2-amino-5- phosphonovaleric acid (D-APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were used as selective antagonists of N-methyl-D-aspartate (NMDA) and non- NMDA receptors, respectively. 2. Ictal-like epileptiform discharges were induced by bath application of the GABA(A)-receptor antagonist picrotoxin. Paroxysmal discharges, 7-25 s in duration, occurred spontaneously or could be evoked by electrical stimulation. These events consisted of an initial paroxysmal depolarizing shift (PDS) followed by a long-duration depolarization (LLD) with superimposed late PDSs. 3. The amplitudes of the initial PDS, LLD, and late PDSs were linearly dependent on membrane potential, increasing with hyperpolarization and diminishing on depolarization. All responses reversed polarity near 0 mV. Under voltage- clamp conditions, both transient and sustained currents were observed, coincident with PDSs and the LLD, respectively. The duration of the ictal- like events was similar under current- and voltage-clamp conditions, suggesting activation of intrinsic membrane currents did not significantly prolong epileptiform discharges. 4. Bath application of D-APV (20 μM) decreased the amplitude and duration of both the initial PDS and LLD without affecting the time-to-onset of epileptiform activity. This antagonist decreased the depolarization associated with the late PDSs and the number of spikes riding on the LLD. Complete suppression of the initial PDS, LLD, or late PDSs was never observed with D-APV application. 5. CNQX decreased the amplitude and duration of the initial PDS and delayed the onset of the initial PDS three- to sixfold. The LLD and late PDSs were virtually abolished by the non-NMDA antagonist. When D-APV was applied in the presence of the non-NMDA antagonist, the CNQX-resistant component was suppressed. 6. A small EPSP-like depolarization was observed to precede the rapid onset of the initial PDS. This depolarization displayed an unconventional voltage dependence, decreasing with hyperpolarization and increasing with depolarization over the range -90 to -50 mV. In the presence of CNQX, early depolarizations were also noted to occur. Such responses decreased in amplitude as the cell was hyperpolarized. The latency-to-onset of PDSs recorded when CNQX was present was voltage dependent, increasing with hyperpolarization and decreasing with depolarization. This early depolarizing event may serve as a trigger for paroxysmal activity. 8. These results indicate that in the immature neocortex, synaptic conductances play a dominant role in generating picrotoxin-induced long-duration ictal-like epileptiform discharges. As in the adult, both NMDA and non-NMDA receptors contribute to paroxysmal discharges in the developing brain. In the immature neocortex, NMDA receptors have a predominant role in acting to initiate epileptiform events, as well as contributing to depolarizations produced by non-NMDA receptors.