Gonadotropin-releasing hormone (GnRH) regulates secretion of the reproductive hormones luteinising hormone (LH) and follicle-stimulating hormone (FSH) in mammals. Homologs of GnRH have been identified in protostomian and deuterostomian invertebrates, and functional studies suggest an ancient role in regulation of reproductive physiology. The aim of this study was to characterise for the first time a GnRH-type signalling system in an echinoderm, using the starfish Asterias rubens as a model experimental system. This is of interest because starfish and other echinoderms i). are deuterostomian invertebrates and therefore they can provide unique insights on the evolution of neuropeptide systems and ii). have a pentaradial body plan without a brain, providing a unique neuroanatomical context for analysis of neuropeptide function. Cloning and sequencing of an A. rubens GnRH-type precursor (ArGnRHP) revealed that it contains a single copy of the putative neuropeptide pQIHYKNPGWGPGamide (ArGnRH). The existence of this peptide in extracts of nerve cords from A. rubens was confirmed by mass spectrometry. Analysis of the expression of ArGnRHP in A. rubens using mRNA in situ hybridisation revealed that it is restricted to cells in both the ectoneural and hyponeural regions of the radial nerve cords and circumoral nerve ring. A cDNA encoding an A. rubens GnRH-type receptor (ArGnRHR) was cloned and sequenced and pharmacological characterisation of this receptor revealed that ArGnRH is its cognate ligand. Furthermore, phylogenetic analysis revealed that this receptor is indeed an orthologue of vertebrate GnRH receptors. Characterisation of the GnRH-type signalling system in A. rubens provides a basis for investigation of its physiological roles in starfish. Discovery of the physiological roles of the ArGnRH in starfish may provide important new insights into the evolution of GnRH neuropeptide systems in the animal kingdom.