In fish, the GH/IGF system installs very early during development suggesting that this system could promote embryonic growth and development. In contrast to mammals, the embryonic growth rate of poikilotherms depends considerably on the incubation temperature. Therefore, the aim of this study was to determine if variations of embryo growth in response to temperature could be associated with modifications in the gene expression of the GH/IGF system. In this study, using whole mount in situ hybridisation, we demonstrated that embryo incubation temperature (4, 8, and 12°C) did not change the timing of GH–1 and GH–2 mRNA expression in somatotroph cells (stage 24). Similarly, at hatching (stage 30), we did not observe an obvious difference in GH protein and GH–1 and GH–2 transcript amounts in relation to the incubation temperature. Furthermore, from stage 22 to 25, the highest temperature led to a specific up-regulation of IGF–2 (2–fold between 4 and 12°C), and both IGF-RIa and IGFRIb mRNA (1.5–fold between 4 and 12°C), while no difference was observed for IGF–1 mRNA. Conversely, at hatching, the highest temperature specifically down-regulated IGF–2 (3fold between 4 and 12°C) and both IGF receptor mRNAs (2 fold between 4 and 12°C) present in the head, while no difference was observed in the trunk. Our results demonstrated that different incubation temperatures during trout embryonic development did not change the stage of somatotroph cell appearance. Before hatching, IGF–2 and both IGF receptors, but not IGF–1 mRNA, were specifically up-regulated by high temperatures and could be related to the enhancement of embryonic growth rate.