1 Historical perspective.- The origin of the receptor concept.- Mass action law and occupancy theory.- The concept of efficacy.- The concept of spare receptors.- Rate theory.- Allosteric theory.- Operational models of pharmacological agonism.- Summary.- 2 Methods for characterization of receptors based on receptor-mediated responses in tissue or intact cell preparations.- Characterization of receptor specificity.- Generalizations regarding the determination of equilibrium dissociation constants (KD values) for receptor-ligand interactions in intact tissue preparations.- Determination of KD values for receptor-agonist interactions, KDA.- Determination of KD values for receptor-partial agonist interactions, KDP.- Determination of KD values for receptor-antagonist interactions, KDB.- Summary.- 3 Identification of receptors using direct radioligand binding techniques.- Methods—data generation.- Choice of a radioligand.- The incubation.- Separation of bound from free radioligand.- Criteria expected for binding of D to the physiological receptor, R.- Data obtained to establish the criteria for a physiologically relevant receptor.- Summary.- 4 Complex binding phenomena.- Mathematical descriptions of complex binding phenomena.- Computer-assisted analysis of complex binding phenomena.- Analysis of receptor subtypes.- Independent data consistent with the existence of receptor subtypes.- Receptor affinity states.- The ternary complex model (TCM) and expansions of the TCM.- Thermodynamic parameters of receptor-ligand interactions.- Summary.- 5 The preparation and study of detergent-solubilized receptors.- General properties of biological membranes and detergent micelles.- Choice of a biological detergent.- Solubilizing receptors from biological membranes.- Methods for analysis ofdetergent-solubilized receptors.- Summary.- 6 The topographical fate of ligand-receptor complexes as reflected by the properties of ligand binding to intact cells.- Biochemical approaches for discriminating between cell surface receptors versus intracellular receptor-ligand complexes.- Biochemical evidence consistent with recycling of cell surface receptors.- Assessment of rate constants for receptor turnover using a steady state mathematical analysis of intact cell radioligand binding data.- The heavy amino acid density-shift technique for quantitating receptor synthesis and turnover.- Summary.