We simultaneously measured neurotransmitter amino acids by the microdialysis technique and cortical CBF by laser-Doppler flowmetry in the ischemic penumbral cortex of rats subjected to 2-h normothermic (36.5-37.5 degrees C) transient middle cerebral artery (MCA) clip-occlusion. Brains were perfusion-fixed 3 days later and infarct volume measured. CBF (% of preischemic values) fell to 32 +/- 2% (mean +/- SD) during ischemia and rose to 157 +/- 68% during recirculation. Extracellular glutamate levels increased from a baseline value of 7 +/- 3 microM to a peak value of 180 +/- 247 microM 20-30 min following onset of ischemia but subsequently returned to near baseline levels after 70 min of ischemia despite ongoing MCA occlusion. The threshold CBF for moderate glutamate release was 48%. Massive glutamate release was seen during the first 60 min of MCA occlusion in the two animals showing the largest infarcts and occurred at CBF values < or = 20% of control levels. Mean CBF during ischemia exhibited an inverse relationship with infarct volume, and the magnitude of glutamate release during ischemia was positively correlated with infarct volume. Extracellular gamma-aminobutyrate and glycine changes were similar to those of glutamate but showed no significant correlation with infarct volume. These results suggest that (a) accumulation of extracellular glutamate is an important determinant of injury in the setting of reversible MCA occlusion and (b) reuptake systems for neurotransmitter amino acids may be functional in the penumbra during transient focal ischemia.