9Elucidation of the important roles played by peptides as hypothalamic-adenohypo- physeal releasing factors, or regulatory hormones, has in recent years led to the recognition that peptides may also be of significance as intercellular messengers in other regions of the nervous system. In this regard, it is interesting that Sub- stance P, which has been proposed as a putative neurotransmitter in the spinal cord, was rediscovered by Leeman and her co-workers during their search for the corticotropin-releasing factor in the hypothalamus. Indeed, with the wide- spread availability and use of radioimmunoassay techniques, it has become ap- parent that various "hypothalamic releasing factors" are localized in extrahypo- thalamic areas of the central nervous system as well. This book represents an expression of the belief that the impact on neurobiology of research into neuro- peptides will be comparable to, if not greater than, the recent achievements obtained with the biogenic amines. As already appears to be the case, future inves- tigations on brain pep tides will undoubtedly uncover a host of new transmitter candidates, with obvious implications for neuropharmacology. Perhaps the most dramatic developments in this field have been the discoveries of the endogenous opiate peptides (enkephalin and endorphin), and the profound physiological and behavioral effects of specific peptides.