Meiosis is an indispensable process of sexual reproduction. However, detailed information on the regulatory mechanisms that initiate meiosis is not available. Progestins are important steroids regulating final maturation in male and female vertebrates. In male teleosts, it is known that progestin induces spermiation and sperm maturation. However, a role for progestin in early spermatogenesis or meiosis has not yet been described. Using several in vitro gonadal culture systems in male and female, we have analyzed the role of 17α, 20β-dihydroxy-4- pregnen- 3- one (DHP), a natural progestin in teleost fish, in controlling spermatogenesis and early oogenesis. DHP and its receptors were present in the testis at an early stage of spermatogenesis. Using eel testicular culture systems, DHP was shown to induce DNA replication in spermatogo- nia. Although 11-ketotestosterone, a known initiator of spermatogenesis, also stimulated DNA synthesis in spermatogonia, antibodies against DHP prevented DNA replication when added during the period in which meiosis was initiated. DHP treat- ment also induced the expression of meiosis specific markers, such as DmcI and Spo11. Furthermore, Spo11 expression and synaptonemal complexes (SC), specific features of the meiotic prophase, were detected in testicular fragments cultured with DHP in some germ cells that showed morphological characteristics of undifferentiated spermatogonia. In huchen and carp ovarian organ culture, DHP induced DNA synthesis of oogonia. In addition, the expression of Spo11 was observed his- tochemically and SC were ultra-microscopically detected in some germ cells. Therefore, DHP is also implicated in the regu- lation of early oogenesis from oogonial proliferation to initiation of the first meiotic division. These data suggest that DHP, a progestin, is an essential factor for the initiation of meiosis in both spermatogenesis and oogenesis.