Sharp waves (SPWs) are thought to play a major role in intrinsic hippocampal operations during states in which subcortical and cortical inputs to hippocampus are reduced. This study describes evidence that such activity occurs spontaneously in appropriately prepared rat hippocampal slices. Irregular waves, with an average frequency of approximately 4 Hz, were recorded from field CA3 in slices prepared from the temporal region of hippocampus. The waves persisted for hours and were not accompanied by aberrant discharges. Multi-electrode analyses established that they were locally generated within each of the subfields of CA3 and yet were coherent between subfields. The sharp waves were reversibly blocked by either cholinergic or serotonergic stimulation. Various lines of evidence indicate that they are propagated by the CA3 associational system.