Spreading depression (SD) elicited in the cerebral cortex of anaesthetized rats does not penetrate into epileptic foci (penicillin) or cortical regions subjected to repetitive electrical stimulation (6--10 Hz, 0.05--0.01 msec). The extent of the SD block, monitored by the absence of the slow potential change and preservation of spontaneous and evoked EEG activity, can be varied in certain limits by changing the stimulus parameters. When the diameter of the block in the parietal cortex is reduced at a time when its temporal boundary is circumvented by an SD wave, propagating in the rostral direction, a strip of cortical tissue between the block and sagittal sulcus is opened for SD which spreads through this route from the frontal to the occipital cortex and starts reverberating around the block. Other methods for eliciting SD reverberation employ removal of an additional block in a section of the circular pathway and directionally biased propagation around the stimulation point. SD reverberation lasting for at least 3 cycles was observed 109 times (median number of completed cycles 5.2, average cycle duration 4.4 +/- 0.1 min). It is suggested that SD reverberation around an epileptic focus can account for slow oscillations of ictal and interictal discharge and for recurrent seizures.