The relationship between calcium influx and the gating of transmitter release was examined at the release face of a cholinergic presynaptic nerve terminal using a technique that allows the simultaneous recording of both calcium channels at the single-channel level and quantal acetylcholine secretion. Acetylcholine release occurred during large inward calcium currents through many simultaneously open channels but was also gated by very small calcium transients, admitting less than 200 ions, when only one channel was open at a time. These findings provide functional support for a highly structured model of the transmitter release face in which the synaptic vesicle release mechanism is closely tethered to one or more presynaptic calcium channels and the opening of only one of these may be sufficient to trigger quantal secretion.