The present article briefly reviews some aspects on the localization and possible functional roles of neuropeptides. It is emphasized that a large number of peptides can be found in the nervous system and that they in many instances occur together with classical transmitters such as acetylcholine and catecholamines in the same neurons. In agreement, functional studies have revealed that they interact in different ways, both synergistically and antagonistically, with the transmitters. In some instances peptides may also have trophic effects. The recent cloning of neuronal peptide receptors has further substantiated a physiological role for these compounds in the nervous system. Moreover, the recent development of peptide antagonists, which pass the blood brain barrier, now opens up new possibilities to elucidate the functional role of neuropeptides and thus of the coexistence phenomenon.