[3H]Kainate bound to chick cerebellar membranes with a KD of 0.6 microM and with an exceptionally high Bmax of 165 pmol/mg of protein. In octylglucoside-solubilised extracts, the affinity of [3H]kainate was reduced (KD = 2.7 microM), but the Bmax was relatively unchanged (130 pmol/mg of protein). The rank potency of competitive ligands was domoate greater than kainate greater than 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) greater than glutamate. Binding sites for alpha-[3H]amino-3- hydroxy-5-methylisoxazolepropionate ([3H]AMPA) were much less abundant, with KD and Bmax values in membranes of 86 nM and 1 pmol/mg of protein, respectively. The affinity of [3H]AMPA binding was also reduced on solubilisation (KD = 465 nM), but there was an increase in the Bmax (1.7 pmol/mg of protein). Quisqualate and CNQX were the most effective displacers of [3H]AMPA binding, but kainate was also a relatively potent inhibitor. However, in contrast to the displacement profile for [3H]kainate, domoate was markedly less potent than kainate at displacing [3H]AMPA. These results suggest that [3H]AMPA binds to a small subset of the kainate sites that, unlike the majority of the [3H]kainate binding protein, which has been reported to be located in the Bergmann glia, may represent neuronal unitary non-N-methyl-D-aspartate receptors.