Intracellular recordings were obtained from neurons in layer II-III of rat frontal cortex maintained in vitro. The role of excitatory amino acid receptors in generation of picrotoxin (PTX)-induced epileptiform activity was investigated with the use of D-2-amino-5-phosphonovaleric acid (D-APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) as selective antagonists of N-methyl-D-aspartate (NMDA) and non-NMDA receptors, respectively. Bath application of PTX resulted in a decrease in evoked inhibitory postsynaptic potentials (IPSPs) in neocortical neurons and a concomitant increase in a polysynaptic late excitatory postsynaptic potential (1EPSP). Epileptiform burst responses, termed paroxysmal depolarizing shifts (PDSs), subsequently developed. Based on response duration, two types of PDSs were identified. Long PDSs were > 100 ms in duration, whereas short PDSs lasted <50 ms. An early depolarizing potential preceded both types of epileptiform burst response. The NMDA receptor antagonist D-APV reduced the peak amplitude and duration of the PDS. D-APV-insensitive portions of the PDS were greatly attenuated or abolished by CNQX. The non-NMDA antagonist also increased the latency to PDS onset and reduced its duration without affecting peak amplitude. CNQX-insensitive components of the PDS, when present, were abolished by D-APV. Short-duration PDSs could be blocked by CNQX. In these neurons, increasing the stimulation strength produced epileptiform responses of reduced amplitude. Under control conditions, PDS amplitude was a linear function of membrane potential, increasing with hyperpolarization and diminishing on depolarization. The early depolarizing response preceding the PDS showed a variable dependence on voltage. Both decreases and increases in amplitude were observed, in different neurons, on membrane depolarization. In the presence of CNQX, PDS amplitude remained linearly related to membrane potential. Early depolarizations preceding the PDS displayed an unconventional voltage sensitivity, increasing with depolarization over the range -100 to -60 mV. We conclude that, in the PTX disinhibition model of epilepsy, both NMDA and non-NMDA receptors contribute to epileptiform burst responses. An NMDA-receptor-mediated early depolarization may serve as a trigger for burst responses. NMDA receptors are also involved in maintenance of the peak of the paroxysmal depolarizations, whereas non-NMDA receptors are necessary for full expression of the PDS.