Free radicals and platelet activating factor (PAF) have been implicated as important mediators in neuronal injury after cerebral ischemia-reperfusion and, particularly, in postischemic hypoperfusion. The electroencephalography (EEG) is a real time reflection of changes in neuronal activity and metabolism. The objective of this study was to investigate the effects of preventive treatment with Ginkgo biloba extract (EGb 761--Tebonin) in cerebral global ischemia and reperfusion in rats using computerized EEG analysis. Ginkgo biloba extract, known to be, in vitro, a free radicals scavanger and a PAF--antagonist, was administrated in dose of 100 mg/kg over 24 hours, for 5 days before and 5 days after cerebral ischemia--reperfusion. The apparition of isoelectric EEG (flat-line) following 4-vessel occlusion was observed after a mean time of 25 sec. in Ginkgo biloba treated rats and after 18 sec. in control rats (p < 0.0015). Computerized spectral analysis of EEG has shown that the percentage of slow waves at 10 minutes after reperfusion was 117% higher in control group than in Ginkgo biloba group (p < 0.015) and the percentage of slow waves after 15 minutes of reperfusion was 100% higher in the control group than in the Ginkgo biloba group (p < 0.02). Five days after cerebral ischemia--reperfusion the percentage of slow waves was unsignificantly higher in the control group than in the Ginkgo biloba group (p > 0.05).