This review summarizes the revolutionary impact of brain peptides on our understanding of the nervous system and then discusses the localization, distribution, synthesis, receptor sites, and possible function of 32 brain peptides. The peptides are discussed in three subgroups: I) the opioid peptides, which include beta-endorphin, the enkephalins, and dynorphin; II) the pituitary releasing hormones, most of which are wide-spread in the brain and include corticotropin-releasing hormone, luteinizing hormone-releasing hormone, somatostatin, and thyrotropin-releasing hormone; and III) a selection of 12 other peptides potentially important for neurological function, including vasopressin, oxytocin, substance P, cholecystokinin, bombesin, neurotensin, renin, angiotensin, vasoactive intestinal polypeptide, neuropeptide Y, calcitonin gene-related peptide, and calcitonin. Within each individual peptide section, the possible physiological roles in anterior pituitary hormone release, blood-flow regulation, feeding behavior, temperature regulation, nociception, memory and learning, and movement are reviewed. Further, where noted, the peptide findings in Huntington's, Alzheimer's, Parkinson's and psychiatric diseases are emphasized.