AMPA receptors (AMPARs) mediate most of the fast excitatory synaptic transmission in the brain. Native AMPARs typically form heterotetrameric complexes and associate with a variety of auxiliary subunits. Stargazin (gamma2) is the prototype and most studied member of the transmembrane AMPA receptor regulatory proteins family, and regulates several properties of AMPARs. The functional properties of AMPA receptors expressed at synapses shape synaptic currents and determine the short-term plasticity of the synaptic response. Determining the factors that control the gating of AMPA receptors is therefore crucial to understanding how neurons process and encode information.Several models have been proposed to explain the mechanism of action of stargazin. However, none of these proposed mechanisms was able to describe all the effects of gamma2 on AMPARs. In order to better understand how stargazin modulates the gating of AMPARs we tested the effect of a family of mutant GluA2 receptors with altered kinetic properties in outside-out patches using fast perfusion.Analysis of the kinetic profile of the mutant receptors in complex with stargazin showed that the ability of stargazin to modulate these properties was dependent on the stability of the desensitised state of the receptor. Mutants with a very stable desensitised state showed no effect on desensitisation and deactivation, whereas receptors with a less stable desensitised state were more profoundly affected by stargazin. The increase in the steady state current induced by stargazin was also inversely correlated to the stability of the desensitised state. These inverse correlations suggest that the interaction between stargazin and the GluA2 AMPA receptor is state dependent. A kinetic model in which stargazin preferentially modulates the active state of the receptor can describe all the experimental data and previously unknown properties of the complex.