NEUROTRANSMITTER release is initiated by influx of Ca2+ through voltage-gated Ca2+ channels(1,2), within 200 mu s of the action potential arriving at the synaptic terminal(3), as the Ca2+ concentration increases from 100 nM to >200 mu M(4). Exocytosis requires high Ca2+ concentration, with a threshold of 20-50 mu M and half-maximal activation at 190 mu M(5,6). The synaptic membrane syntaxin(7,8), 25K synaptosome-associated protein (SNAP25)9, and vesicle-associated membrane protein (VAMP)/synaptobrevin(10-12), are thought to form a synaptic core complex which mediates vesicle docking and membrane fusion(13-19) Synaptotagmin may be the low-affinity Ca2+-sensor(20-24), but other Ca2+-sensors are involved(25-27) as residual neurotransmission persists in synaptotagmin-null mutants. Syntaxin binds to N-type Ca2+ channels(7,8,28,29) at a site in the intracellular loop connecting domains II and III30. Here we describe Ca2+ dependent interaction of this site with syntaxin and SNAP25 which has a biphasic dependence on Ca2+, with maximal binding at 20 mu M free Ca2+, near the threshold for transmitter release. Ca2+-dependent interaction of Ca2+ channels with the synaptic core complex may be important for Ca2+-dependent docking and fusion of synaptic vesicles.