In order to identify possible relationships between insulin, Insulin-like Growth Factors (IGFs) and the various transcription factors involved in cell proliferation and muscle growth (such as MyoD, myogenin, myostatin), we combined in vivo and in vitro experiments. Changes in food regimen in rainbow trout (Onchorhynchus mykiss) altered the IGF system at peptide and receptor level. The expression of myogenin and myostatin was also altered. IGF expression and muscle growth were partially restored by refeeding. In preliminary experiments in zebrafish (Danio rerio) muscle, ectopic overexpression of IGF-I increased MyoD and myogenin expression. By Western Blot we studied the effects of insulin, IGF-I and IGF-II on the two main signalling pathways in muscle using primary cultures of rainbow trout and seabream (Sparus aurata) muscle cells. IGFs activated both pathways and their efficiency depended on the culture stage, and human and fish peptides had similar effect. Incubation with specific inhibitors showed that wortmannin decreased Akt phosphorylation stimulated by hIGFII, while treatment with PD98059 reduced the activation of MAPK. The role of insulin and IGFs in metabolic processes was studied in seabream myocytes and compared with results in trout. Both IGFs showed a similar effect on glucose uptake stimulation, which decreased when the cells were incubated with wortmannin, confirming that the PI3K/Akt pathway is important for this process in muscle. Insulin and IGFs also stimulated alanine uptake. To find the best markers for fish growth and quality we did preliminary experiments using c-met. Its expression was high during the first stages of development and decreased as differentiation progressed.