Exploration of the mechanisms and plasticity of synaptic transmission has been hindered by the lack of a method to measure single vesicle turnover directly in individual presynaptic boutons at isolated nerve terminals. Although postsynaptic electrical recordings have provided a wealth of invaluable basic information about quantal presynaptic processes, this approach has often proved difficult to apply at most central nervous system synapses. Here we describe the direct optical detection of single quantal events in individual presynaptic boutons of cultured hippocampal neurons. Using the fluorescent dye FM 1-43 as a tracer for presynaptic endocytosis, we have characterized both evoked and spontaneous components of presynaptic function at the level of individual quanta. Our results are consistent with quantal interpretations of previous electrophysiological analyses and provide new information about the unitary membrane recycling event and its coupling to individual action potential stimuli, about spontaneous vesicle turnover at individual boutons, and about the numbers of vesicles recycling at individual boutons.