1. The subcellular distributions of [3H]-alpha-amino-3-hydroxy-5- methylisoxazolepropionate ([3H]-AMPA) and [3H]-kainate binding sites in rat hippocampus were investigated by cell fractionation techniques. 2. Two major populations of [3H]-AMPA sites were detected with the majority of binding located intracellularly in the microsomal (P3) fraction. Most of the remaining sites were in the synaptosomal membrane fraction but some were also present in the nuclear fraction. In contrast, essentially all of the [3H]-kainate binding sites were in the synaptosomal membrane fraction. 3. Saturation binding analysis yielded KD and Bmax values for [3H]-AMPA of 147 nM and 2.6 pmol mg-1 protein respectively for the synaptosomal membrane-associated sites and 129 nM and 5.3 pmol mg-1 protein respectively for the microsomal sites. 4. Both main populations of [3H]-AMPA sites displayed the same rank order of inhibition by competitive ligands, the apparent Mr values of GluR1 subunits were equivalent, suggesting the same degree of post-translational modification and the hydrodynamic properties of the receptor complexes were identical. 5. These data are consistent with the hypothesis that the movement of AMPA receptors between cellular compartments in the postsynaptic neurone could constitute one mechanism underlying long-term potentiation in the hippocampus.