The role of Ca in transmitter release and facilitation has been examined at the squid giant synapse by evaluating the effect of changes in the external concentration of Ca on the excitatory postsynaptic potential (EPSP). Transmitter release followed the fourth power of external Ca at low stimulus frequencies. However, at higher stimulus frequencies, which caused facilitation of the EPSP, transmitter release was found to be related to a lower power of the external Ca concentration. It is proposed that the release site has four Ca receptors, each of which opens a "gate" and that all four gates must be open to activate transmitter release. In addition, it is proposed that after Ca binding the closue of the gates is slow and, hence, that facilitation is due to the partial activation of release sites by a subthreshold one to three Ca ions.