Gonadal development is precisely regulated by the two gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Much progress on understanding the functions of LH and FSH signaling on gonad development has been achieved in the past decades, mostly from studies in mammals, especially genetic studies in both mouse and human. The functions of both LH and FSH signaling in nonmammalian species are still largely unknown. In recent years, using zebrafish, a teleost phylogenetically distant from mammals, we and others have genetically analyzed the functions of gonadotropins and their receptors through gene knockout studies. In this review, we will summarize the pertinent findings and discuss how the actions of gonadotropin signaling on gonad development have evolved during evolution from fish to mammals.