MOST reported actions of kainate are mediated by AMPA (alpha-amino-3-hydroxy-5-methyl 3-isosazolepropionate) receptors(1,2). Here we report that, unlike AMPA which stimulates, kainate elicits a dose-dependent decrease in L-glutamate release from rat hippocampal synaptosomes and also depresses glutamatergic synaptic transmission. Brief exposure to kainate inhibited Ca2+-dependent [H-3]L-glutamate release by up to 80%. Inhibition was reversed by kainate antagonists but not by the AMPA-selective noncompetitive antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3- benzodiazepine (GYKI 52466)(3-7). A corresponding reversible kainate-evoked depression of NMDA (N-methyl-D-aspartate) receptor-mediated excitatory postsynaptic currents (e.p.s.cs) was observed when AMPA receptors were blocked by GYKI 52466. The synaptic depression was preceded by a brief period of enhanced release and a small inward current was also observed. The effects of kainate were unaffected by metabotropic glutamate (mGlu), GABA(A), GABA(B), glycine and adenosine receptor antagonists. These results indicate that glutamate release can be modulated directly by kainate autoreceptors.