Some electrographic seizures are generated intracortically. The cellular and ionic bases of cortically generated spontaneous seizures are not fully understood. Here we investigated spontaneously occurring seizures consisting of spike-wave complexes intermingled with fast runs in ketamine-xylazine anesthetized cats, using dual intracellular recordings in which one pipette contained a control solution and another pipette contained blockers of K(+), Na(+), or Ca(2+) currents. We show that closely located neocortical neurons display virtually identical fluctuations of the membrane potential during electrographic seizures, thus directly demonstrating a high degree of focal synchrony during paroxysmal activity. In addition to synaptic drives, the persistent Na(+) current [I(Na(p))] and probably the high-threshold Ca(2+) current contributed to the generation of paroxysmal depolarizing shifts (PDSs) during cortically driven seizures. Ca(2+)-activated K(+) current [I(K(Ca))] took also part in the control of the amplitude and duration of PDSs. The hyperpolarizing components of seizures largely depended on Cs(+)-sensitive K(+) currents. I(K(Ca)) played a significant, while not exclusive, role in the mediation of hyperpolarizing potentials related to EEG "waves" during spike-wave seizures. We conclude that intrinsic cellular factors have significant role in the generation of depolarizing and hyperpolarizing components of seizures.