Epilepsy is characterized by highly synchronized paroxysmal bursts of activity within a large population of cortical neurones. Because such spontaneous, synchronized discharges can occur even in isolated blocks of neocortex, mechanisms for initiating and coordinating this activity must reside within the cortex itself. However, the specific cellular properties and local neural circuitry responsible for such behaviour are unknown. In a previous study of neocortex in vitro, we found that treatment with the convulsants penicillin and bicuculline led to synchronized bursts which were driven by unusually large and long-lasting excitatory synaptic conductances. I now report evidence that synchronized bursts are initiated by a small, spacially discrete subpopulation of cells located in the area comprising layer IV and upper layer V. Neural elements in these layers appear to project paroxysmal synaptic excitation radially, onto the neurones of other layers.