Growth hormone (GH) is involved in the control of various physiological functions in teleosts, including not only body growth but also reproduction, osmoregulation, metabolism and immunity. Most data on GH regulation come from investigations in mammals and studies in teleosts suggest important species-related variations. We developed specific tools to investigate GH regulation in the European eel: purification of pituitary GH, obtention of specific antibodies and development of homologous radioimmunoassay (RIA), cloning of pituitary GH cDNA and obtention of a specific probe for messenger RNA assays, development of an eel pituitary cell culture system for short- and long-term studies of CH synthesis and release.