n-Aequorin J, a luminescent protein which responds to calcium concentration changes in the order of several hundred micromoles, was injected into the preterminal fiber in the squid giant synapse. The activation of the presynaptic terminal leading to release of transmitter was accompanied by light emission at well-defined sites at the active zone in the presynaptic terminal. Location of these light emission sites was very much the same from one stimulus to the next, indicating that light emission was triggered by the inward calcium current occurring at specific and invariant locations. The distribution, size and number of these QEDs (quantum emission domains) coincides well with the location and number of active zones in the presynaptic terminal. The results imply that transmitter release is triggered by very well-localized calcium concentration changes that may be as high as several hundred micromoles.