The Japanese eel has two characteristics advantageous for the study of the mechanisms controlling spermatogenesis. One is the possibility of artificial induction of the complete process of spermato-genesis from spermatogonial proliferation to spermiogenesis by exogenous gonadotropin injection, and the other is the possibility of inducing this process in an in vitro testicular organ culture or germ-Sertoli cell coculture system. Using the eel system, we analyzed the control mechanisms of spermatogenesis. In Japanese eel, the whole process of spermatogenesis is regulated by several sex steroid hormones. Spermatogonial stem cell renewal is promoted by estradiol-17β (the natural estrogen in vertebrates). Spermatogonial proliferation can be induced by 11-ketotestosterone, the main androgen in teleost. IGF-I is necessary for the action of 11-ketotestosterone in the initiation of spermatogenesis. The action of 11-ketotestosterone is mediated by other factors, such as activin B, produced by Sertoli cells. Although 11-ketotestosterone also induce meiosis and spermiogenesis, the control mechanisms of these processes are not clear. After spermiogenesis, immature spermatozoa undergo sperm maturation, thereby becoming capable of fertilization. Sperm maturation is regulated by 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DP), which is progestogen in teleosts. The 17α,20β-DP acts directly on spermatozoa to activate the carbonic anhydrase existed in the spermatozoa. This enzymatic activation causes an increase in the seminal plasma pH, enabling spermatozoa to motile.