Rats were injected unilaterally with colchicine in the dentate gyrus of the dorsal hippocampus. Two weeks later, under urethane anesthesia, extracellular recordings were obtained on both sides from pyramidal neurons of the CA1 and of the CA3 regions of the dorsal hippocampus. Microiontophoresis was used to assess the responsiveness of these neurons to kainate, glutamate and ibotenate. The colchicine injection produced a nearly complete destruction of the granule cells of the ipsilateral dentate gyrus and of their mossy fiber projections to CA3 without apparently affecting other hippocampal neurons. On the lesioned side, the potency of kainate in activating CA3 pyramidal neurons was reduced by 94% compared to the same neurons on the intact side. However, the excitatory effect of glutamate was unchanged and that of ibotenate only slightly reduced. Kainate was 80 times more potent in activating CA3 than CA1 pyramidal neurons on the intact side, whereas this ratio had dropped to 2.6 on the lesioned side. The selective decrease of the effectiveness of kainate in activating CA3 pyramidal neurons following the colchicine lesion suggests that this amino acid, but not glutamate and ibotenate, produces most of its excitatory effect in the intact CA3 region by releasing (an) excitatory neurotransmitter(s) from mossy fibers terminals, the nature of which remains to be identified.