We investigated the actions of a specific N-methyl-D-aspartate (NMDA) antagonist, 2-amino-5-phosphonovaleric acid (2-APV), on bicuculline-induced epileptogenesis in organotypic explant cultures from neonatal rat neocortex. Explants were maintained in roller tubes for 3-5 weeks. The late, plateau phase of the intracellularly recorded paroxysmal depolarization shift was sensitive to both intracellularly injected hyperpolarizing currents and 2-APV, suggesting that this component is generated by a voltage-dependent, regenerative process that is mediated by activation of NMDA receptors. The results support the hypothesis that NMDA receptors play an important role in the generation of epileptiform activity by localized circuits of neocortical neurons.