The technology of drug testing is rapidly advancing. This book brings together world renowned experts in the fields of pharmacology and physiology united with the common theme of describing methods, theoretical concepts and in vitro systems used to study drug receptor mechanisms and the action of drugs on receptors. The first section deals with the physiology and pharmacology of integrated natural systems, the second with the new theoretical ideas used to model receptor systems (i.e. the nature of efficacy, receptor activation) and the third with the new in vitro systems available for the study of receptors and drugs. The latest ideas regarding receptor theory are critically evaluated and presented to the reader.