1. The presynaptic action of kainate (KA) receptor activation at the mossy fibre-CA3 synapse was examined using fluorescence measurement of presynaptic Ca2+ influx as well as electrophysiological recordings in mouse hippocampal slices. 2. Bath application of a low concentration (0.2 microM) of KA reversibly increased the amplitude of presynaptic volley evoked by stimulation of mossy fibres to 146 +/- 6 % of control (n = 6), whereas it reduced the field excitatory postsynaptic potential (EPSPs) to 30 +/- 4 %. 3. The potentiating effect of KA on the presynaptic volleys was also observed in Ca2+-free solution, and was partly antagonized by (2S, 4R)-4-methylglutamic acid (SYM 2081, 1 microM), which selectively desensitizes KA receptors. 4. The antidromic population spike of dentate granule cells evoked by stimulation of mossy fibres was increased by application of 0.2 microM KA to 160 +/- 10 % of control (n = 6). Whole-cell current-clamp recordings revealed that the stimulus threshold for generating antidromic spikes recorded from a single granule cell was lowered by KA application. 5. Application of KA (0.2 microM) suppressed presynaptic Ca2+ influx to 78 +/- 4 % of control (n = 6), whereas the amplitude of the presynaptic volley was increased. 6. KA at 0.2 microM reversibly suppressed excitatory postsynaptic currents (EPSCs) evoked by mossy fibre simulation to 38 +/- 9 % of control (n = 5). 7. These results suggest that KA receptor activation enhances the excitability of mossy fibres, probably via axonal depolarization, and reduces action potential-induced Ca2+ influx, thereby inhibiting mossy fibre EPSCs presynaptically. This novel presynaptic inhibitory action of KA at the mossy fibre-CA3 synapse may regulate the excitability of highly interconnected CA3 networks.