This book is being published at a time when opioid receptors have recently been cloned. The structural characteristics of opioid receptors and the recent advances in their molecular cloning and expression are explicated. Connecting these cloned opioid receptors with the pharmacology of opioid receptor actions is of particular importance. The use of antisense oligodeoxynucleotides to opioid receptors to elucidate the neural mechanisms of opioid action is detailed. As analgesia is one of the most important clinical functions for opioids and opioid peptides, this book concentrates on the detailed mechanisms of action of opioids and opioid peptides for the production of analgesia. The significant pharmacologic actions of opioids and opioid peptides in gastrointestinal function, immune function, cardiovascular and respiratory regulation, addiction and the cellular mechanisms involved in tolerance and dependence of opioids are also described. The book will be of interest both to researchers investigating the neural mechanisms of the pharmacologic and physiologic functions of the endogenous opioid system and to clinicians who are involved in pain management and assessment.