1. Spontaneous non-synaptic epileptiform activity was induced by bathing rat hippocampal slices in low-Ca(2+) solution. Extracellular recordings from electrodes placed on both sides of a complete cut showed that non-synaptic activity was synchronized across the lesion. 2. Ion-selective electrode recordings showed that each event was accompanied by a transient increase in extracellular potassium that diffused across the lesion. The synchrony was destroyed when a thin film was inserted into the lesion site. 3. Local pressure ejection of KCl evoked an event that subsequently propagated across the lesion. 4. After a complete lesion was made, afterdischarges evoked on one half of a slice were not detected on the other half. 5. Voltage-sensitive dye imaging methods showed that epileptic activity propagated across the mechanical lesion without significant attenuation or additional delays. The velocity of the activity was consistent with that of the slow diffusion of a potassium wave. 6. Since field effects were significantly attenuated across the lesion and all gap junctions and cell processes across the lesion would be cut, these data show that extracellular diffusion, most probably potassium, is sufficient to synchronize populations of neurons and propagate slow frequency epileptiform activity.