✓ An increase in extracellular K ⁺ concentration ([K ⁺ ] _e ) of the rat hippocampus following fluid-percussion concussive brain injury was demonstrated with microdialysis. The role of neuronal discharge was examined with in situ administration of 0.1 mM tetrodotoxin, a potent depressant of neuronal discharges, and of 0.5 to 20 mM cobalt, a blocker of Ca ⁺⁺ channels. While a small short-lasting [K ⁺ ] _e increase (1.40- to 2.15-fold) was observed after a mild insult, a more pronounced longer-lasting increase (4.28- to 5.90-fold) was induced without overt morphological damage as the severity of injury rose above a certain threshold (unconscious for 200 to 250 seconds). The small short-lasting increase was reduced with prior administration of tetrodotoxin but not with cobalt, indicating that neuronal discharges are the source of this increase. In contrast, the larger longer-lasting increase was resistant to tetrodotoxin and partially dependent on Ca ⁺⁺ , suggesting that neurotransmitter release is involved. In order to test the hypothesis that the release of the excitatory amino acid neurotransmitter glutamate mediates this increase in [K ⁺ ] _e , the extracellular concentration of glutamate ([Glu] _e ) was measured along with [K ⁺ ] _e . The results indicate that a relatively specific increase in [Glu] _e (as compared with other amino acids) was induced concomitantly with the increase in [K ⁺ ] _e . Furthermore, the in situ administration of 1 to 25 mM kynurenic acid, an excitatory amino acid antagonist, effectively attenuated the increase in [K ⁺ ] _e . A dose-response curve suggested that a maximum effect of kynurenic acid is obtained at a concentration that substantially blocks all receptor subtypes of excitatory amino acids. These data suggest that concussive brain injury causes a massive K ⁺ flux which is likely to be related to an indiscriminate release of excitatory amino acids occurring immediately after brain injury.