Binding of [3H](R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ([3H]AMPA) to quisqualate receptors in the presence of SCN- ions produced curvilinear Scatchard plots. Kinetic investigations of [3H]AMPA binding showed that the curvilinearity cannot be explained by assuming binding to two separate binding sites or by considering it due to cooperative interaction. A more likely explanation is that the quisqualate receptors exist in two states, one with high and one with low affinity for [3H]AMPA. Chaotropic ions change the relaxation constant between the two states.