Minimal excitatory postsynaptic potentials were evoked in CA3 pyramidal neurons by activation of the mossy fibres in hippocampal slices from seven- to 16-day-old rats. Conditioning intracellular depolarizing pulses were delivered as 50- or 100-Hz bursts. A statistically significant depression and potentiation was induced in four and five of 13 cases, respectively. The initial state of the synapses influenced the effect: the amplitude changes correlated with the pretetanic paired-pulse facilitation ratio. Afferent (mossy fibre) tetanization produced a significant depression in four of six inputs, and no significant changes in two inputs. Quantal content decreased or increased following induction of the depression or potentiation, respectively, whereas no significant changes in quantal size were observed. Compatible with presynaptic maintenance mechanisms of both depression and potentiation, changes in the mean quantal content were associated with modifications in the paired-pulse facilitation ratios, coefficient of variation of response amplitudes and number of response failures. Cases were encountered when apparently "presynaptically silent" synapses were converted into functional synapses during potentiation or when effective synapses became "presynaptically silent" when depression was induced, suggesting respective changes in the probability of transmitter release. It is concluded that, in juvenile rats, it is possible to induce lasting potentiation at the mossy fibre-CA3 synapses by purely postsynaptic stimulation, while afferent tetanization is accompanied by long-lasting depression. The data support the existence not only of a presynaptically induced, but also a postsynaptically induced form of long-term potentiation in the mossy fibre-CA3 synapse. Despite a postsynaptic induction mechanism, maintenance of both potentiation and depression is likely to occur presynaptically.