The effects of sex steroids and pituitary extract on the development of gonads as well as gonadotropin-II (GTH-II) pituitary content in the aquacultured Japanese eels (Anguilla japonica) were investigated. Gonadosomatic index (GSI) was not significantly stimulated in females after 3 injections over 6 weeks, nor in males after 6 injections over a 12-week period of steroids (estradiol-17β, E2; testosterone, T; 5α-dihydrotestosterone, OHT; for each steroid with 2 doses, 0.75 and 3.75 mg/kg body weight). Salmon pituitary extract (10 mg/kg body weight) induced a progressive increase in GSI after 3-, 6-, 9- and 14-injections over a 14-week period. E2 and pituitary extract significantly increased pituitary GTH-II content in a dose-dependent manner. Androgens also increased pituitary GTH-II content but the effects were much weaker than E2. Pituitary GTH-II content were significantly increased in maturing eels (GSI > 20%) injected with salmon pituitary extract. It is suggested that the salmon pituitary extract effects on GSI in the cultured yellow eels was a direct effect on ovaries, and a multiple effect of sex steroid on various levels of reproduction.