Recently, Caldwell et al. (2013) reported the function of both IGFs in nutrition regulation during compensatory growth in rainbow trout. In the same species, Li and Leatherland (2008), in a study on the transition from embryo to early juvenile, showed the importance of the metabolic role of IGF system during that developmental phase. Overall, these findings contributed to recognize that IGFs in fish were doing in part, in addition to its mitogenic effects, some metabolic functions normally attributed to insulin. Through vertebrate's evolution, insulin has acquired more importance in the regulation of metabolism while IGFs have focused on the control of growth. With these first studies, it is possible to answer some of the original questions on IGFs function in fish muscle, but new aspects appeared to be solved in relation to the transduction pathways of insulin and IGF-I receptors where the in vitro model could provide valuable information.