By using microdialysis extraction of (3)H-D-aspartate and concomitant recordings of extracellular direct current (DC) potentials, the effect of middle cerebral artery occlusion (MCAO) was studied continuously over a period of 100 min in the cerebral cortex of rats. From analysis of the DC potentials, rats subjected to MCAO could be divided into three groups, one in which the dialysis probe was located in the ischemic core, one in which the probe was in the penumbra, and one in which the probe was in nonischemic tissue. In general, extraction of (3)H-D-aspartate was positively correlated with the DC potential; i.e., changes in the extraction were concurrent with changes in the DC potential. Comparing the different animal groups by integration of all extraction values obtained during MCAO over time, (3)H-D-aspartate extraction was reduced by 40% in the penumbra, and by 58% in the ischemic core, compared with the sham-operated controls. No changes was found in the nonischemic group. In the penumbra group, extraction of (3)H-D-aspartate was reduced initially upon institution of MCAO but recovered to control-like levels over a period of 15-40 min, despite ongoing MCAO. In addition, extraction was reduced transiently during periinfarct depolarizations. A mean of all extraction values obtained during MCAO in the penumbra group was reduced by 47% compared with a mean of values obtained before institution of MCAO. Induction of death resulted in a reduction of (3)H-D-aspartate extraction by 86%. The present results provide direct evidence that uptake of Glu is reduced both in the ischemic core and in the penumbra of the cerebral cortex following MCAO in rats, possibly contributing to the initiation and spread of infarction. The results further indicate that uptake of Glu in the penumbra recovers to control-like levels, despite ongoing MCAO, providing evidence that Glu uptake by the Glu transporter proteins is reinstituted and/or up-regulated.