An understanding of the physiology and mechanisms involved in the regulation of growth is essential in an aquaculture context to improve production. Understanding intra-population variability will improve the use of molecular markers in the development of selection programs. The main goal of this study was to deepen our understanding on regulators in which a selection ultimately acts. To do so, we used individuals issued from a breeding program launched in our facilities decades ago on brook trout Salvelinus fontinalis issued from a feral anadromous population. Thirteen selected and sixteen control families belonging to the fifth generation of this program were sampled (N = 4471 individuals including 2078 selected and 2398 controlled). At 1 of age, all the fish were weighed and measured. An analysis of the average weight of each family was done to determine the family performance in terms of growth. The four best and four worst performing families were chosen. Within each family, the eight individuals with the highest and the eight individuals with the lowest weights were sacrificed. The pituitary gland, brain, liver and white muscle were removed and placed in sterile tubes, frozen in liquid nitrogen, and stored at -80°C. In this tissue, we will evaluate the variability of the expression of some genes acting in the growth mechanism loop (brain: neuropeptide Y, somatostatin, and ghreline; pituitary: growth hormone GH and its receptor GHr; liver and muscle: GHr, IGF1 and IGF1r). The hypothesis tested is that gene expression levels of NPY, GH, GHr, IGF-1, IGF-1, IGF-1 r, somatostatin, and ghrelin will be higher in individuals from selected families thus enabling them to have greater growth potential. Finally, these results will be validated in relation to the weight-length data, calculating the Fulton condition factor (K) to determine the degree of well-being or robustness.