In summary, we have presented evidence that neuropeptides can function as either positive or negative growth regulatory factors during development. The ACTH family of peptides appear to act predominantly as a positive growth regulatory factor - enhancing neurite outgrowth, cell survival, biochemical maturation and behavioral expression. These effects of ACTH are most pronounced prior to the time the afferent cell has reached its target. Thus, ACTH may act as a low level general neurotrophic growth regulatory factor. The opioids have the opposite effect. These neuropeptides inhibit neurite extension, cell survival, and biochemical maturation. The effects of these negative growth regulatory factors are observed when the afferents have reached their targets. The action of the opioids is thought to occur through specific receptors and known second messenger systems. Thus, CNS neuropeptide levels can have important actions in regulating the development of a variety of CNS systems, and permanently influencing the structure and function of the brain.