In order to understand the mechanisms implicated at the hypothalamo-pituitary level in growth-reproduction interaction in salmonids, the gonadotropin-releasing hormone (GnRH) action on growth hormone (GH) release was studied, in rainbow trout (Oncorhynchus mykiss). In vivo, acute treatment with salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II) and an sGnRH analogue [(DArg6Pro9)sGnRH] was performed on catheterized fish. The different forms of GnRH have no effect on plasma GH levels of immature and mature fish, but induce a stimulation of gonadotropin (GtH) release in mature fish. In the present work we have adapted and validated a culture system for GH regulation studies. In this system, increasing doses of sGnRH, (DArg6Pro9)sGnRH and cGnRH-II are inactive on GH release (24 hr incubation) in immature or mature fish, but stimulate GtH release in a dose-dependent manner, sGnRH (10(-6) M) has no action on time (15 min-24 hr). In a perifusion system, sGnRH also has no action on GH release but stimulates GtH release. The present results obtained using in vivo and in vitro techniques adapted for GH regulation studies, show that GnRH does not function as a growth hormone-releasing factor in rainbow trout as it does in goldfish.