The plasma levels of gonadotropin (GtH) II, the major pituitary hormone controlling final gamete maturation in fish were compared between wild and captive broodstocks of striped bass (Morone saxatilis) in order to gain an understanding of the failure of captive fish to undergo final oocyte maturation (FOM) and spermiation. Captive fish, both male and female, had lower plasma GtH II levels during the spawning season, and this seems to be the cause of the reproductive dysfunction in striped bass, and presumably in other fishes reared in captivity. Injections of an agonist of gonadotropin-releasing hormone (GnRHa) were effective in inducing pituitary GtH II release, which in turn can initiate FOM and spermiation. However, a prolonged presence of elevated GtH II in the circulation was necessary to induce complete FOM, and long term production of milt. A sustained elevation of plasma GtH II was achieved using two polymeric, sustained-release GnRHa-delivery systems, a treatment which resulted in 100 per cent FOM, ovulation and spawning of viable eggs, and production of good quality milt for up to 21 days after treatment. This novel technology was shown to be effective in a variety of commercially important marine fishes.