Long-term potentiation at CA3-CA1 hippocampal synapses exhibits an early phase and a late phase, which can be distinguished by their underlying molecular mechanisms. Unlike the early phase, the late phase is dependent on both cAMP and protein synthesis. Quantal analysis of unitary synaptic transmission between a single presynaptic CA3 neuron and a single postsynaptic CA1 neuron suggests that, under certain conditions, the early phase of LTP involves an increase in the probability of release of a single quantum of transmitter from a single presynaptic release site, with no change in the number of quanta that are released or in postsynaptic sensitivity to transmitter. Here, we show that the cAMP-induced late phase of LTP involves an increase in the number of quanta released in response to a single presynaptic action potential, possibly due to an increase in the number of sites of synaptic transmission between a single CA3 and a single CA1 neuron.