Although the functional and anatomical properties of synaptic connections hold essential clues about central nervous system processing, basic synaptic properties, such as the number of functional and anatomically identified release sites per connection and the release probability per site, remain unknown at most cortical synapses. Silver et al. combined in vitro electrophysiology with light and electron microscopy to investigate the quantal parameters of synaptic transmission between layer 4 spiny stellate cells and layer 2/3 pyramidal cells of rat somatosensory cortex. They observed a one-to-one relationship between the number of anatomically identified synaptic contacts and the number of functional release sites. The concentration of glutamate in the synaptic cleft following release was independent of release probability, the probability of release at each identified synaptic contact was extremely high, and the release sites operated independently. The properties are exactly what are required for reliable transmission of spatially distributed, timing-based signals.