Controversy exists regarding the site of modification of synaptic transmission during long-term plasticity in the mammalian hippocampus. Here we used a fluorescent marker of presynaptic activity, FM 1-43, to directly image changes in presynaptic function during both short-term and long-term forms of plasticity at presynaptic boutons of CA3–CA1 excitatory synapses in acute hippocampal slices. We demonstrated enhanced presynaptic function during long-term potentiation (LTP) induced either chemically (with tetraethylammonium), or by high-frequency (200-Hz) electrical stimulation. Both of these forms of LTP required activation of L-type voltage-gated calcium channels and NMDA receptors in the postsynaptic CA1 neuron. These results thus implied that a long-lasting increase in the efficacy of synaptic transmission is likely to depend, at least in part, on enhanced transmitter release from the presynaptic neuron.