Excitatory synaptic interactions between CA3 neurones in slices from guinea-pig hippocampus were examined. Recurrent excitatory post-synaptic potentials (e.p.s.p.s) were evoked by action potentials in a single presynaptic neurone or by the antidromic activation of part of the CA3 pyramidal cell population. The peak amplitude of unitary e.p.s.p.s was 1-2 mV at potentials between -64 and -70 mV. Their time to peak was 7-12 ms and the initial phase of their decay was slower than that of a somatically injected voltage pulse. Recurrent e.p.s.p.s were often followed by a small (0.3 mV) hyperpolarization, or undershoot. Recurrent e.p.s.p.s were compared with e.p.s.p.s evoked by stimulating mossy fibres, which terminate proximally on apical dendrites of CA3 pyramidal cells. They were of slower time course and reversed at a more positive potential than mossy fibre e.p.s.p.s. Some synaptic terminals made by recurrent axon collaterals apparently terminate at distant locations on apical dendrites. The decay of both recurrent e.p.s.p.s and dendritic voltage pulses was prolonged by membrane depolarization within a 10-15 mV subthreshold potential range. Voltage-dependent inward currents activated by the synaptic depolarization may contribute to the slow initial decay of these synaptic events. The undershoot did not occur when transmission of a unitary e.p.s.p. failed and was of slower time course than the hyperpolarization due to an inhibitory post-synaptic potential (i.p.s.p.). It was suppressed by intracellular application of K+ channel blockers and probably reflects an intrinsic outward current activated as a consequence of the synaptic depolarization. Considerable temporal summation of synaptic potentials occurred when recurrent synapses were activated twice at an interval of 5-10 ms, typical of the spontaneous burst firing pattern of CA3 neurones. The mean facilitation of a second e.p.s.p. at this interval was about 0.6. The efficacy of a third and subsequent e.p.s.p.s at similar interval was reduced. Presynaptic bursts of three to five action potentials evoked summed e.p.s.p.s of amplitude 2-4 mV, with time to peak 20-40 ms and decaying phase of similar duration. Their rising phase was relatively smooth and summed events were succeeded by an undershoot. Presynaptic bursts could cause a post-synaptic neurone to discharge.