The catecholamines epinephrine and norepinephrine evoke specific beta-adrenergic responses in a variety of tissues.Examples of processes modulated by these agonists are chronotropic and inotropic cardiac responses, relaxation of smooth muscle, and lipolysis in adipose tissue.The facts that beta-adrenergic responses are limited to specific tissues and that there exist stereospecific constraints, i.e., the naturally occurring (-)-isomers of the catecholamines are more potent than the (+)-isomers, imply a recognition system based on stereocomplementarity (Gilbert and Greenberg, 1984).These observations, based on adrenergic responses, reinforce one of the underlying tenets of pharmacology and therapeutics: The specific actions of hormones and neurotransmitters result from high-affinity, stereospecific interactions with tissues.The concept of an entity or substance that recognizes and discriminates on the basis of geometric properties of hormones or drugs has been evolving for more than a century (Langley, 1905; Dale,1906).This proposed moiety has been functionally designated “receptor.” Receptors are defined by their ability to recognize hormones or drugs of a specific class through direct binding interactions and, of equal importance, translate the binding interactions and, of equal importance, translate the binding event into a biological response.