Using whole-cell recording from CA1 hippocampal pyramidal neurons and minimal stimulation of Schaffer collaterals, we have studied what seem to be single synapses. Although the transmission at a putative single synapses is quite unreliable, the synapse can be made to release transmitter reliably in response to the second stimulus in a pair of stimuli that re presented in rapid succession (e.g., 50 ms separation). Statistical analysis of transsmision failures seen with such paired pulse stimulation reveals that the majority of stimulus-evoked synaptic currents (> 90%) are produced by a single synapse under the conditions of minimal stimulation, even if multiple synapses are actually present. Individual synapses appear to release either zero or one quantum; that is, a single synapse seems to have only one functional release sit at any time. After the release site has been used, approximately 20 ms is required to refill the site so that it can be used again.