The release of endogenous glutamate from guinea-pig cerebrocortical synaptosomes evoked by dendrotoxin, beta-bungarotoxin, and 4-aminopyridine is compared. Dendrotoxin and 4-aminopyridine cause Ca2+-dependent release, representing a partial depletion of the KCl-releasable transmitter pool. The decrease in the plasma membrane potential caused by 4-aminopyridine or dendrotoxin and the evoked release of glutamate from a transmitter pool accord with the inhibitory action of these agents on certain K+ conductances. In contrast, the massive release of glutamate evoked by beta-bungarotoxin is produced in the presence of Ca2+ but not of Sr2+, a result consistent with a generalised permeabilisation of synaptosomal plasma membranes. Although dendrotoxin inhibits the binding of beta-bungarotoxin and the resultant synaptosomal lysis, demonstration of a direct effect of beta-bungarotoxin binding per se on K+ permeability is impractical owing to its phospholipase A2 activity.