The initial landmarks in opioid receptor research were the demonstration of stereospecificity (Becket and Casey, 1954 ) and the identification of receptor sites in central nervous system (CNS) membrane preparations (Pert and Snyder, 1973 ; Simon et al., 1973 ). Subsequent behavioral studies with a number of opiate agonists suggested that more than one opioid receptor population was present in the CNS (Martin et al., 1976 ; Gilbert and Martin, 1976 ). The concept of multiple opioid receptors has been extensively investigated, and present data indicate that three major opioid receptor populations are present in the brain (Iwamoto and Martin, 1981 ; Zukin and Zukin, 1981 ; Wood, 1982 ). These are the mu (μ), delta (δ), and kappa (κ) sites.