The control mechanism of gonadotropin-releasing hormone (GnRH) on gonadotropin (GTH) release was studied using cultured pituitary cell or cultured whole pituitary obtained from Testosterone (T) treated and control immature rainbow trout. The release of FSH was not changed by salmon type GnRH (sGnRH), chiken-II type (cGnRH-II), GnRH analogue ([des-$Gly^{10}D-Ala^6$] GnRH ethylamide) and GnRH antagonist ([Ac-3, 4-dehydro-$Pro^1$, D-p-F-$Phe^2$, D-$Trp^{3,6}$] GnRH) in cultured pituitary cells of T-treated and control fish. Indeed, FSH release was not also altered by sGnRH in cultured whole pituitary. All tested drugs had no effect on the release of LH in both culture systems of control fish. The levels of LH, in contrast, such as the pituitary content, basal release and responsiveness to GnRH were increased by T administration in both culture systems. In addition, the release of LH in response to sGnRH or cGnRH-II induced in a dose-dependent manner from cultured pituitary cells of T-treated fish, but which is not significantly different between in both GnRH at the concentration examined. Indeed, LH release was also increased by sGnRH in cultured whole pituitary of T-treated fish. GnRH antagonist suppressed the release of LH by sGnRH ($10^{-8};M$) and GnRH analogue ($10^{-8};M$) stimulation in a dose-dependent manner from cultured pituitary cells of T-treated fish, and which were totally inhibited by $10^{-7};M$ GnRH antagonist. These results indicate that the sensitivity of pituitary cells to GnRH is elevated probably through the T treatment, and that GnRH is involved in the regulation of LH release. GnRH-stimulated LH release is inhibited by GnRH antagonist in a dose-dependent manner. The effects of gonadal steroids on FSH levels are less clear.