Furosemide, a chloride cotransport inhibitor, reversibly blocked synchronized burst discharges in hippocampal slices without reducing the pyramidal cell response to single electrical stimuli. Images of the intrinsic optical signal acquired during these slice experiments indicated that furosemide coincidentally blocked changes in extracellular space. In urethane-anesthetized rats, systemically injected furosemide blocked kainic acid-induced electrical discharges recorded from cortex. These results suggest that (i) neuronal synchronization involved in epileptiform activity can be dissociated from synaptic excitability; (ii) nonsynaptic mechanisms, possibly associated with furosemide-sensitive cell volume regulation, may be critical for synchronization of neuronal activity; and (iii) agents that affect extracellular volume may have clinical utility as antiepileptic drugs.