The neural ganglion of the ascidian Ciona intestinalis regenerates in its entirety within a few weeks of ablation. Here we investigate the role of gonadotropin–releasing hormone–like immunoreactive (GnRH–li) cells in regeneration. Immunocytochemical studies show that in addition to a previously described plexus of GnRH–like neurones located in association with the dorsal strand, the normal adult brain contains GnRH–li neurones. These are predominantly localized to the ventral cortical rind at the posterior of the ganglion. Following ablation, non–process bearing GnRH–li cells appear in the regenerating area within two days. By day 8 post–ablation, process bearing GnRH–li cells are detected close to the regenerating brain. The number of these cells increases at later stages. GnRH–li cells are first detected within the regenerating brain at 14 days post–ablation and their number subsequently increases. These cells are initially concentrated along the ventral surface of the regenerating brain near to the dorsal strand. Double labelling studies with bromodeoxyuridine show that none of the GnRH–li cells are labelled at any stage of regeneration. The data are consistent with a sub–population of the new neurones being derived from GnRH–li neuroblasts born prior to ablation, which migrate from the dorsal strand complex into the regenerating ganglion.