We have explored the mechanisms involved in the facilitation of glutamate release mediated by the activation of kainate receptors in the rat hippocampus using isolated nerve terminal (synaptosome) and slice preparations. In hippocampal nerve terminals, kainate produced an increase of glutamate release at concentrations of agonist ranging from 10-1000 µM. In hippocampal slices, kainate at low nanomolar concentration (20-50 nM) also produced an increase of evoked excitatory postsynaptic currents (eEPSCs) at mossy fibre/CA3 synapses. In both, synaptosomes and slices the effect of kainate was antagonized by CNQX, and persisted after pretreatment with a cocktail of antagonist for receptors whose activation could potentially have produced facilitation of release. These data indicate that the facilitation of glutamate release observed is mediated by the activation of presynaptic glutamate receptors of the kainate type. Mechanistically, the observed effects of kainate appear to be the same in synaptosomal and slice preparations. Thus, the effect of kainate on glutamate release and mossy fibre/CA3 synaptic transmission was occluded by the stimulation of adenlylyl cyclase by forskolin and suppressed by the inhibition of protein kinase A by H-89 or Rp-Br-cAMP. We conclude that kainate receptors present at presynaptic terminals in the rat hippocampus mediate the facilitation of glutamate release through a mechanism involving the activation of an adenylyl cyclase/second messenger cAMP/ protein kinase A signalling cascade.