SNARE-complex assembly mediates synaptic vesicle fusion during neurotransmitter release.Although many SNARE proteins are available per vesicle, only one to three SNARE complexes are minimally needed for a fusion reaction.Here, we use high-resolution measurements of synaptic transmission in the calyx-of-Held synapse from mutant mice in which syntaxin-1 is either constitutively open and SNARE-complex assembly is enhanced or syntaxin-1 level is largely reduced, to examine the relation between SNARE-complex assembly and neurotransmitter release.