Traditionally, the two major systems of intercellular communication (i.e. the nervous and endocrine systems) were considered separate functional and anatomical entities. Recent studies have provided evidence that the biochemical elements of these systems have common early phylogenetic origins and have suggested that, with the exception of their anatomical diversity, all the systems of intercellular communication are biochemically similar. On the basis of these findings, we suggest that the overlaps between the nervous and endocrine systems. the widespread tissue production of hormones, and other phenomena are now more easily understood.