One of the key questions in epileptology is whether or not active epileptic foci are able to increase excitability or to facilitate epileptic activity of other brain regions. The aim of the present investigations was therefore to test whether epileptogenesis of the CA1 region of the hippocampus is changed with synchronous synaptic input. In hippocampus slices (500mm) of rats (220-280g) epileptic activity was elicited by withdrawal of Mg2+ from the bath fluid. Electrical activity was monitored by conventional field potential recordings. After 90 min synchronous epileptiform activity in the CA3 and CA1 regions epileptic activity persisted in both areas with functional separation from each other (section by a water jet). Separation before withdrawal of Mg2+ led to non- appearance of epileptic activity in CA1. With the latter protocol epileptic activity appeared in CA1 when Schaffer collaterals had been stimulated electrically before functional separation. In addition to direct synaptic also other mechanisms have to be taken into account since electrical stimulation was effective with complete deafferentation of CA1.