The effects of permanent focal ischemia on specific proteins of the cerebral hemisphere were studied by unilateral occlusion of the middle cerebral artery (MCAO) in rat. Brain proteins were prepared 72 h after the occlusion and analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The proteins were identified by their interaction with rabbit antibodies against rat serum proteins and anti-transferrin antibodies. SDS-PAGE analysis of the proteins prepared from ischemic tissue showed significant increase in the 66 and 80 kDa components; where a marked decrease in the 260 kDa protein occurred in the ischemic and para-ischemic tissues. The 66 kDa and 80 kDa proteins stained intensely with anti-serum protein antibodies, indicating that they are related to plasma components. Moreover, the 66 kDa band had the same electrophoretic mobility as bovine serum albumin used as a standard molecular size marker. The 80 kDa band was identified as transferrin by staining with the specific antibody. Transferrin was immunolocalized in the penumbra of cerebral cortex, hippocampal CA1 region and dentate gyrus of the ischemic cerebral hemisphere. The present results suggest that alteration in the brain content of 66 kDa (albumin), 80 kDa (transferrin) and 260 kDa (unidentified) proteins may reflect early effects of focal ischemia.