Several different peptides are widely but precisely distributed in central, peripheral, and autonomic nervous systems. Although their functional importance in many of their sites remains to be understood, their occurrence in specific cells strongly suggests that they serve as neurotransmitters or as neuromodulators. 1 , 2 , 3 At present, at least 33 peptides have been described in neurons and nerve terminals within mammalian CNS, other than those related to endocrine function which may be considered as neurotransmitters or neuromodulators. Although their discovery has added a totally new dimension to our analysis of neural activity, the inaccessibility and diffuse localization of central peptidergic pathways have made it difficult to identify many peptides as neurotransmitters. However, the presence of several different peptides in the peripheral nervous system has made it possible to carry out experiments necessary to determine whether these peptides can be designated as transmitters. This group of putative neurotransmitters in peripheral neurons includes somatostatin, substance P, vasoactive intestinal polypeptide, and neurotensin. 2 , 4 , 5 , 6 If a peptide is released as a transmitter or a modulator substance from a nerve terminal, it would be expected that its terminal store would be maintained by axoplasmic transport of the peptide. Evidence for axoplasmic transport of certain neuropeptides has been provided by accumulation of im- munoreactive peptides proximal to the site of peripheral nerve ligation or crush. 4 , 6 , 7 (For review see References 2 and 5.)