AMPA receptors (AMPARs) mediate fast excitatory transmission at a majority of central synapses. The single-channel properties of recombinant AMPARs depend on RNA editing, splice variation and subunit composition – and are characterised by multiple conductance openings (Swanson et al. 1997). Individual AMPARs function as homomeric or heteromeric assemblies of four subunits, each of which can bind glutamate (Rosenmund et al. 1998). The conductance state adopted by recombinant AMPA/kainate chimeric receptors (GluR3/6; Rosenmund et al. 1998) and GluR2-containing AMPARs in cerebellar granule cells (Smith & Howe, 2000) depends on the agonist concentration used to activate the receptor. In the present study, we investigated whether this concentration-dependent behaviour extends to unmodified recombinant homomeric GluR3 (Ca2+-permeable) and heteromeric GluR1/2 (Ca2+-impermeable) receptors. Additionally, we examined the concentration dependence of native AMPARs in cells that express these subunits, namely CA1 hippocampal cells. Recombinant AMPAR channels were examined in Chinese Hamster Ovary (CHO) cells transiently transfected with GluR3 or Glu1/2 AMPA receptor subunits. Native AMPAR-channels were examined in CA1 cells in 250μm hippocampal slices from P12 rats humanely killed. Single-channel currents were recorded in outside-out patches exposed to glutamate (200nM-20mM). Homomeric GluR3 AMPARs displayed 3 discrete conductance states of approximately 8.5, 15 and 23pS (n = 14 patches). The proportion of openings reaching the highest conductance state increased with glutamate concentration, from 23.8 ± 7.2% (n = 4) in 0.1mM glutamate to 62.8 ± 6.4% (n = 4) in 10mM glutamate (mean ± S.E.M.). It seems likely that the different conductance states represent the binding of 2, 3 or 4 ligands to GluR3 respectively (see Rosemund et al. 1998). In CA1 cells, the single-channel events displayed up to four conductance states. Increasing glutamate concentrations from 200nM to 20mM revealed a similar concentration dependence of channel conductance. Our experiments provide evidence that unmodified recombinant homomeric GluR3 and heteromeric GluR1/2 receptors adopt a channel conductance that depends on the concentration of agonist activating the receptor. AMPARs in hippocampal CA1 cells in slices display similar behaviour. Thus it seems probable that this property is conserved across the AMPA receptor family.