The action of phytochemicals when administered to fish for sex reversal and as a growth booster is a recent subject of study. The present research is focused on the use of Tribulus terrestris (TT) extract (Zygophylaseae), and 17? methyltestosterone (MT) to investigate theirs potential action on growth performance, body composition and masculinization of rainbow trout ( Oncorhynchus mykiss).   In a completely randomized design, experimental diets supplimented with 2.5, 5, 10 and 20 (g/kg food) TT and two diets containing 3 and 6 (mg/kg food) MT along with a control diet (free of TT and MT) were evaluated. The seven treatments, each consisted of three replicate. The experiment was conducted on the first feeding, genetically all-female rainbow trout with the mean weight of   0.11±0.02 g. A stock of American imported fish were assigned to experimental diets for 3 different trials of 60, 90 and 120 days. At the end of each experiment, fish performance was assesed in terms of mean final weight, specific growth rate (SGR), condition factor (CF), food conversion ratio (FCR), weight gain percentage (WGP) and body composition. Histological examinations was carried out to determine the developmental stage of testes and ovary. The results indicate that   MT₆-treated fish exhibited successful growth acceleration compared to the control. TT_2.5   treatment also improved the growth rate of fish. At the termination of the experiment all treatments except TT₁₀ and TT₂₀ showed a better specific growth rate (SGR) compare to control. Food conversion ratio (FCR) of MT and TT- treated fish were significantly greater than that of control at the end of 60 days of experiment (p<0.05). Codition factor (CF) was not significantly different from Control group in all 3 Trials (p>0.05). It was concluded that usage of higher doses and treatment duration of TT extract   had negative effects on growth performance. In addition, It could be concluded that the MT and TT treatments had no negative effects on body composition. Nevertheless, the highest and the lowest percentage crude lipid were observed in MT₃ (8.32) and TT₂₀ (7.44) treatments, respectively. Mean percentage moisture and ash were 77.29±1.08 and 1.69±0.18, respectively. On the other hand, the highest and the lowest percentage crude protein were observed in MT₃ (14.73) and control (14.19).   Histological studies of sampled fish at the end of 90 and 120 days of treatments did not show a clear gonadal structure diagnosis, so fish sex was undetectable. Therefore, gonad examination, 14 months after hatch, was carried out again only for our first trial (60 days). Macroscopic and microscopic investigations revealed that TT treatments (in considered dosages) had no effects on masculination of rainbow trout. Nevertheless, MT treatments (3 and 6 mg/kg food) resulted in 88.88 and 71.42 percent male population, respectively. Gonads from control, TT and MT treatment were in an early stage and completely normal. Furthermore, among the fish subjected to MT₃, one of the treated individuals were found to develop intersex gonads.

  Key words: Rainbow trout Oncorhynchus mykiss, Tribulus terrestris, 17? methyltestosterone, Growth performance, Masculinization.