Introduction: Fish are useful models for physiological studies in which using nutritional, growth and hormonal indices can lead to the understanding of several biological mechanisms in vertebrates. In this study, the effects of the addition of two amino acids, L-arginine and L-ornithine, to a commercial feed on the growth indices and the blood levels of insulin-like growth factor-I (IGF-I) of rainbow trout, Oncorhynchus mykiss were examined. Materials and Methods: The fish (average initial weight 45±4 g) were fed for 8 weeks with one of the following 4 dietary treatments: Commercial trout diet supplemented with 2% L-arginine-(T1) 2% L-ornithine-(T2) 2% L-arginine + 1% L-ornithine-(T3) 3% L-arginine + 1% L-ornithine-(T4) and the commercial feed without addition of the amino acids (controls). Results: According to the results, maximum weight gain (268.94±5.84%) and specific growth rate (SGR) (2.33±0.05% day–1) were observed in the fish of group T3, while maximum hepatosomatic index (HSI) (1.49±0.04%) was recorded in group T4. These amounts were significantly higher than their counterpart indices of the control group (P<0.05). Supplementing the feed with the amino acids, overall, increased IGF-I levels throughout the study period. At the end of the eighth week, in the T3 and control groups maximum and minimum IGF levels were 1180 pg/ml and 980.35 pg/ml respectively. Conclusions: Results of this study showed the positive effects of providing fish diet with additional amino acids, especially a combination of arginine and ornithine, on growth promotion in rainbow trout.