In the present study, the effects of 17β-estradiol (E2) treatment on the expression of preprosomatostatin (PPSS) I, PPSS II′, and PPSS II" mRNA in the hypothalamus and endocrine pancreas (Brockmann body), as well as the effects of E2 treatment on plasma somatostatin (SS)-14 and -25 concentrations in sexually immature rainbow trout ( Oncorhynchus mykiss), were investigated. E2treatment significantly ( P < 0.001) depressed both plasma SS-14 and SS-25. In the hypothalamus, E2 treatment significantly ( P < 0.001) decreased the levels of PPSS I and PPSS II" mRNA. However, there was no effect of E2treatment on PPSS II′ mRNA levels. In the pancreas, E2treatment had no significant effect on the levels of either PPSS II′ mRNA or PPSS II" mRNA. However, E2 treatment significantly ( P < 0.005) decreased levels of PPSS I mRNA. These data suggest that E2 acts, in part, to increase plasma growth hormone levels in rainbow trout by decreasing the endogenous inhibitory somatostatinergic tone by inhibiting plasma levels of both SS-14 and SS-25 and hypothalamic levels of mRNA encoding these proteins.