Under cultivation conditions, male Japanese eels (Anguilla japonica) have immature testes containing only spermatogonia together with inactive testicular somatic cells, Leydig cells and Sertoli cells. Using a recently developed organ culture system for eel testes, we have shown that hormonal induction of spermatogenesis in eel testes involves gonadotropin stimulation of Leydig cells to produce 11-ketotestosterone, a potent androgen in fish. In turn, 11-ketotestosterone activates Sertoli cells to stimulate premitotic spermatogonia to complete spermatogenesis. Our current research focuses on the isolation and characterization of genes that show altered expression in eel testes during gonadotropin-induced spermatogenesis. One up-regulated and three down-regulated genes have been isolated. Northern blot analysis and in situ hybridization reveal that mRNA for activin B is absent in testes before gonadotropin injection and is abundant in Sertoli cells in testes injected with gonadotropin for one to six days after injection. This stimulation of activin B mRNA is accompanied by spermatogonial proliferation. Gonadotropin treatment also causes a rapid rise in the testicular concentrations of mRNA for 3 beta-hydroxysteroid dehydrogenase, the rate-limiting enzyme for gonadotropin-induced 11-ketotestosterone production. We have also obtained three down-regulated cDNAs which are abundant in testes before gonadotropin treatment and disappear almost completely in testes one day after gonadotropin injection.