Radioligand-binding experiments have demonstrated that both D-1 (adenylate cyclase stimulatory) and D-2 (adenylate cyclase inhibitory) dopamine receptors demonstrate high- and low-affinity agonist-binding states. Under appropriate conditions, both D-1 and D-2 high-affinity binding states can be labeled directly with 3H-labeled agonists. Previous studies that identified a portion of high-affinity 3H-labeled agonist binding as a distinct dopamine receptor subclass (D-3 or autoreceptor) were, in fact, labeling the high-affinity agonist-binding state of postsynaptic D-1 receptors.