In order to clarify the role of gonadal sex steroids in the synthesis of gonadotropin (GTH) subunits in immature rainbow trout, we examined in vitro and in vivo effects of testosterone (T) on the pituitary mRNA levels of GTH I $eta$, GTH II$eta$ and a subunits by Northern blot analysis and on the pituitary content levels of GTH I$eta$ and GTH II$eta$by radioimmunoassay (RIA). The mRNA levels of the a subunit in T-treated fish were not changed more dramatically than those in control fish both in vivo and in vitro. Interestingly, the mRNA levels of GTH I$eta$ in T-treated fish were shown to be slightly lower than those in the control fish under these experimental conditions, but no differences were observed in pituitary GTH I$eta$ contents. In contrast, the mRNA levels and pituitary contents of GTH II$eta$ subunit were strongly increased by T both in vivo and in vitro. These results demonstrate that the expressions of GTH I$eta$ and II$eta$ subunit genes in immatue rainbow trout pituitary are subjected to differential regulation by T.