In order to understand the mechanisms underlying seizure generation, the present study investigated the functional and anatomical interaction between seizures and interictal bursts. Bathing rat brain slices of the hippocampus and entorhinal cortex in zero magnesium medium elicited electrographic seizures. In the later period, the electrographic seizures were eventually replaced by the interictal bursts. In this phase, lowering [K+]o and raising [Ca2+]o restored the tonic seizures by suppressing the interictal bursts. A multiple knife cut experiment revealed that the seizures arose in the entorhinal cortex and the interictal bursts originated in CA3. A selective knife cut across the subiculum showed that the interictal bursts, propagating from CA3 to the entorhinal cortex, disrupted and suppressed the electrographic seizures in standard Mg(2+)-free medium.