Temporal lobe seizures in humans correlate with stereotyped electrophysiological patterns that can be reproduced in animal models to study the cellular and network changes responsible for ictogenesis. Seizure-like discharges that mimic seizure patterns in humans were induced in the entorhinal cortex of the in vitro isolated guinea pig brain by 3-min arterial applications of the GABA(A) receptor antagonist bicuculline. The onset of seizure is characterized by a paradoxical interruption of firing for several seconds in principal neurons coupled with both enhanced interneuronal firing and increased extracellular potassium (Gnatkovsky et al. 2008). The evolution of action potential features from firing break to excessive and synchronous activity associated with the progression of seizure itself is analyzed here. We utilized phase plot analysis to characterize action potential features of entorhinal cortex neurons in different phases of a seizure. Compared with preictal action potentials, resumed spikes in layer II-III neurons (n = 17) during the early phase of the seizure-like discharge displayed 1) depolarized threshold, 2) lower peak amplitude, 3) depolarized voltage of repolarization and 4) decelerated depolarizing phase, and 5) spike doublettes. Action potentials in deep-layer principal cells (n = 8) during seizure did not show the marked feature changes observed in superficial layer neurons. Action potential reappearance correlated with an increase in extracellular potassium. High-threshold, slow-action potentials similar to those observed in the irregular firing phase of a seizure were reproduced in layer II-III neurons by direct cortical application of a highly concentrated potassium solution (12-24 mM). We propose that the generation of possibly nonsomatic action potentials by increased extracellular potassium represents a crucial step toward reestablish firing after an initial depression in an acute model of temporal lobe seizures. Resumed firing reengages principal neurons into seizure discharge and promotes the transition toward the synchronized burst firing that characterizes the late phase of a seizure.