This paper reports the pathological changes and MDA content of the brain tissue in the early stage of cerebral ischemia due to middle cerebral artery occlusion (MCAO) in Wistar rats. The neurones showed changes of the rough endopiasmic reticulum and the mitochondrium as early as 2 minutes after MCAO. Under the light microscope, "ischemsc cells" with shrunken and condensed cytoplasm and nucleus appeared 10 minutes after MCAO. The end-product of lipid peroxidation, MDA, in the isch. emic brain tissue also significantly increased 2 minutes after MCAO, being correlated synchronously with the ultrastructural changes of the neurones. This suggested that free radicals play a role in the early stage of brain injury due to ischemia.