The synapse plays pivotal roles in various neuronal functions, with its efficacy determining the opening and closing of neuronal circuits. Among the parameters that determine synaptic efficacy, presynaptic factors are of crucial importance. However, these factors are among those that are the least well studied. Patch-clamp recording from the calyx of Held presynaptic terminal, described in this chapter, has enabled us to address directly questions about the presynaptic regulatory mechanisms in synaptic transmission.