1. Cortical spreading depression (SD) is a propagating transient suppression of electrical activity associated with depolarization, which may contribute to the pathophysiology of important neurological disorders, including cerebral ischemia and migraine. The purpose of this study is to ascertain whether SD propagation depends on local accumulation of extracellular K+ or glutamate. 2. Propagating SD recorded through microdialysis probes perfused with artificial cerebrospinal fluid (ACSF) was much smaller than that recorded with conventional glass microelectrodes, presumably because some SD-induced transient changes in the extracellular fluid composition were buffered by ACSF. We have exploited this effect to determine whether perfusion with a medium containing increasing amounts of K+ and/or glutamate favors SD propagation. 3. Increasing the concentration of K+ (15-60 mmol/l) in the perfusion medium dose-dependently restored SD propagation, whereas application of 100-250 mumol/l glutamate through the microdialysis probe had no effect. Superimposing 200 mumol/l glutamate onto 15 and 30 mmol/l K+ did not further improve the restoration of SD propagation by K+. 4. Because potent uptake mechanisms may efficiently clear exogenous glutamate from the extracellular space, the effect of local inhibition of high-affinity glutamate uptake was also studied. Perfusion of the recording microdialysis probe with 1 mmol/l L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC), either alone or together with 200 mumol/l glutamate, had no effect. In addition, L-trans-PDC did not potentiate the positive effect of 30 mmol/l K+ on SD propagation. 5. These results strongly suggest that high extracellular K+, and not extracellular glutamate, is the driving force sustaining SD propagation.(ABSTRACT TRUNCATED AT 250 WORDS)