Until about 10 yr ago, it was generally assumed that the nervous system, the endocrine system, and the neuroendocrine system are not only anatomically distinct, but that they differ also with respect to the chemical messengers they use. There existed, however, a few exceptions, such as the presence of noradrenaline (NA) in the adrenal medulla or of dopamine (DA) in the external layer of the median eminence. It has since become apparent, however, that these exceptions reflect the general rule in the sense that many hormones are now known to occur in nerve cells— neuroendocrine hormones occur in both nerve cells and endocrine cells, and, more rarely, neurotransmitters occur in endocrine and neuroendocrine cells. In addition, a large number of previously unknown peptides has been found to occur at practically all levels of the nervous, endocrine, and neuroendocrine systems, thereby leading to a major conceptual change in the field of cell-to-cell communication through chemical signaling. Thus, although the anatomical distinctions between the three systems are still valid, the chemical specificity no longer holds. It appears that, depending on their location, several of the newly discovered peptides can function as either neurotransmitters, hormones, or neuroendocrine regulators.