This project aims to determine the function of Anti-Müllerian hormone (AMH) in fish as a factor in testis determination, development, and maturation at puberty. AMH, also named Müllerian inhibiting substance or spermatogenesis preventing substance, belong s to the transforming growth factor-ß family. In mammals, AMH is secreted from testicular Sertoli cells. In the foetus it directs Müllerian duct regression and controls spermatogenesis by suppression of Leydig cells until puberty and may be associated wit h precocious puberty . AMH is present in fish, where it has been shown to block spermatogenesis at a very early stage by preventing the differentiation of rapidly proliferating spermatogonia. We hypothesise that AMH suppresses the expression and/or activi ty of the receptor responsible for mediating the effects of 11-ketotestosterone.