Calcium entry through presynaptic calcium channels controls the release of neurotransmitter. It is not known whether the putative calcium sensor that triggers this rapid neurotransmitter release is close enough to be activated by the large increase in the Ca2+ concentration (calcium 'domain') reached within nanometres of a single calcium channel or whether many channels have to open. We tested this in a calyx-type synapse in the rat medial nucleus of the trapezoid body. We compared the quantal content of postsynaptic currents with the presynaptic calcium current that flows during an action potential, and the results suggest that more than 60 calcium channels open for each vesicle that is released. In addition, we dialysed terminals with the slow calcium buffer EGTA, which reduced phasic transmitter release at concentrations as low as 1 mM. These results indicate that the distance that calcium ions must diffuse to reach the calcium sensor is relatively long, and that therefore Ca2+ entry through multiple calcium channels is needed to release a vesicle.