Radiollgand binding is a straightforward technique that measures the bindmg of a labeled agonist or antagonist to its receptor. It is applicable to a variety of receptor preparations, ranging from purified receptors to tissue slices, or even whole animals. However, membranes or broken-cell preparations are undoubtedly the most widely used. Radioligand binding allows the affinity of drugs for their receptors to be determined very readily and It also allows the number of receptors in a tissue or cell to be quantified-something that was impossible before the mtroductlon of binding techniques. Furthermore, the technique can be adapted to study the association and dissociation kinetics of llgand binding (1,2), as well as complex allosteric mteractlons between ligands (1) or between receptors and effector molecules, such as guanine nucleotide-binding proteins (G proteins) (1,2).