EEGs were recorded with depth electrodes in 8 monkeys undergoing transient middle cerebral artery ligation. Electrodes measured EEG, cerebral blood flow (CBF), and tissue oxygen simultaneously during and after occlusion. An EEG frequency analysis was performed. Electrode sites were examined microscopically, and infarction size, tissue vacuolization index, and neuronal morphology were described quantitatively. Serial neurological examinations were performed. Two patterns of EEG frequency change were delineated, dependent upon degree of ischaemia. Mild ischaemia, as indicated by less severe clinical deficits, higher CBF during occlusion, and minor pathological changes was associated with large increases in slow EEG activity and decreases in fast EEG activity during occlusion, with recovery of slow activities to baseline, but continued suppression of fast activities 24 h later. Severe ischaemia was associated with suppression of both fast and slow frequencies during occlusion, with slow activities returning to baseline and fast activities remaining suppressed 24 h later. The best quantitative EEG indicator of severity of ischaemia was suppression of slow wave activity during occlusion. The best EEG indicator that an ischaemic event had occurred 24 h previously was continued suppression of fast EEG activities. These data may be helpful in the design of EEG frequency analysis studies for monitoring the time course of human cerebral ischaemia and for retrospective diagnosis of transient ischaemic attacks (TIAs).