Abstract: The amino acid sequences of the three endogenous opioid peptide precursors are known, and the anatomical distribution of the opioid peptides has been studied extensively. This report summarizes these anatomical studies and looks at the problems that result from the biochemical relatedness of the precursors. We also discuss the relationship of opioid systems to opioid receptors, and the use of anatomical studies to derive new hypotheses of opioid function and provide dynamic measures of opioid neural activity, especially via specific mRNA quantitation.