In the rapidly developing area of neuroimmunology, primary at- tention has been given thus far to the bidirectional exchange of information between the immune system and the neuroendocrine system. Several recent texts on this subject are based alm ost exclu- sively on experimental and clinical data obtained in mammalian species. One of the interesting insights gained from these studies is that the chemical messenger substances used in this exchange by either system are the same, or at least closely related to each other. Cytokines, primarily thought of as being dispatched by cells of the immune system, have recently been identified to be products of neural, in particular, glial elements. Conversely, neuropeptides for- merly considered to be characteristic of the neuroendocrine apparatus are now known to be manufactured and dispatched by immuno- regulatory cells as weIl. The present text is the first to provide an overview of the rich body of information obtained in aseries of broadly based comparative studies. This work in representatives of higher invertebrates has opened new vistas, in particular, regarding the immunomodulatory role played by opioid and other neuropeptides as signal molecules in the ceIl-mediated internal regulation of the immune system.