The wide range of actions of the FMRFamide-related peptides and the biological importance of peptidergic messengers.

The importance of peptides as intercellular messengers is discussed. The view is put forward that peptides evolved early in evolution as chemical messengers and that they have come to exert a wide range of actions. Using as an example the FMRFamide (Phe-Met-Arg-Phe-NH2) related peptide family of molluscs, the wide range of peptide actions on membrane currents is discussed and considered in relation to co-localization of peptides with low molecular weight (or "classical") intercellular messengers.