Binding of [3H]kainate to the kainate subtype of excitatory amino acid receptors revealed two binding sites. The high-affinity site (Kd = 3.5 nM) was sensitive to calcium ions and has a molecular target size as determined by high-energy radiation inactivation analysis of 76,600 daltons. The low-affinity site (Kd = 65 nM) was insensitive to calcium ions and has a molecular target size similar to the quisqualate-subtype of excitatory amino acid receptors. The change in binding parameters of the two sites with radiation dose strongly suggests that low affinity, calcium-insensitive [3H]kainate binding sites are equivalent to quisqualate sites.