Changes in the latency of small excitatory postsynaptic potentials were observed in association with induction of long-term modifications of synaptic transmission in slices of rat neocortex and guinea-pig hippocampus. After potentiation response latency decreased in 3/10 cases in the neocortex and in 6/24 cases in the hippocampus, and increased after depression in 4/8 cases in the neocortex. These latency changes could not be attributed to changes in presynaptic fibre excitability, monosynaptic inhibition, release kinetics or activation kinetics of postsynaptic ion channels. We conclude therefore that potentiation led to the activation of previously silent synapses of fast-conducting afferents and depression to the inactivation of previously functional synapses. Thus, neocortical and hippocampal synapses can be in a non-functional state, and regimes that induce long-term potentiation and depression not only change the efficacy of synapses but also alter their functional state.