Inhibition of transmitter release by presynaptic receptors is widespread in the central nervous system and is typically mediated via metabotropic receptors; In contrast, very little is known about facilitatory receptors, and synaptic activation of a facilitatory autoreceptor has not been established. Here we show that activation of presynaptic kainate receptors can facilitate transmitter release from hippocampal mossy fiber synapses. Synaptic activation of these presumed ionotropic kainate receptors is very fast (<10 ms) and Lasts for seconds. Thus, these presynaptic kainate receptors contribute to the shortterm plasticity characteristics of mossy fiber synapses, which were previously thought to be an intrinsic property of the synapse.