Extracellular [K+] can increase during some pathological conditions, resulting into excessive glutamate release through multiple mechanisms. We here investigate the overflow of [3H]D-aspartate ([3H] D-ASP) and of endogenous glutamate elicited by increasing [K+] from purified rat cerebrocortical synaptosomes. Depolarization with [K+] <or= 15 mmol/L provoked [3H] D-ASP and glutamate overflows almost totally dependent on external Ca2+. Consistent with release by exocytosis, the overflow of [3H] D-ASP evoked by 12 mmol/L K+ was sensitive to clostridial toxins. The overflows evoked by 35/50 mmol/L K+ remained external Ca2+-dependent by more than 50%. The Ca2+-independent components of the [3H] D-ASP overflows evoked by [K+] > 15 mmol/L were prevented by the glutamate transporter inhibitors DL-threo-beta-benzyloxyaspartate (DL-TBOA) and dihydrokainate. Differently, the overflows of endogenous glutamate provoked by [K+] > 15 mmol/L were insensitive to both inhibitors; the external Ca2+-independent glutamate overflow caused by 50 mmol/L KCl was prevented by bafilomycin, by chelating intraterminal Ca2+, by blocking the mitochondrial Na+/Ca2+ exchanger and, for a small portion, by blocking anion channels. In contrast to purified synaptosomes, the 50 mmol/L K+-evoked release of endogenous glutamate or [3H]D-ASP was inhibited by DL-TBOA in crude synaptosomes; moreover, it was external Ca2+-insensitive and blocked by DL-TBOA in purified gliosomes, suggesting that carrier-mediated release of endogenous glutamate provoked by excessive [K+] in CNS tissues largely originates from glia.