In the presence of convulsant drugs such as picrotoxin, neurons in the hippocampal-slice preparation generate synchronized depolarizing bursts. This synchrony occurs on a time scale of tens of milliseconds and is produced by excitatory synaptic interactions between neurons. The synaptic interactions themselves occur on a time scale of tens of milliseconds. The "epileptiform" local-field potential during such synchronized bursts is comb-shaped ("ringing"), whereas the field potential expected if action potentials in neighboring neurons were uncorrelated is noisy and not comb-shaped. This suggests that individual action potentials are locally synchronized on a time scale of 1 ms. We have previously shown, using computer simulations, that electrical interactions-mediated by currents flowing in the extracellular medium-can plausibly explain action-potential synchronization in experiments where chemical synapses are blocked. The present simulations demonstrate that electrical interactions can also account for action-potential synchronization-and thus the "ringing" shape of the field potential-during epileptiform bursts, where excitatory synapses are functional. The field potential is thus a modulating influence on, as well as a reflection of, underlying neuronal transmembrane events. © 1985.