To assess the existence of central amino acid sensing systems in fish we carried out two experiments in rainbow trout. In the first one, we injected ICV two different branched-chain amino acids (BCAA), leucine and valine, and assessed food intake up to 48 h later. Leucine decreased and valine increased food intake. In a second experiment, 6h after similar ICV treatment we determined changes in parameters related to putative amino acid sensing systems. Amino acid sensing systems respond to leucine in hypothalamus and telencephalon, and to valine in telencephalon. The decreased food intake observed in fish treated ICV with leucine could relate to changes in mRNA abundance of hypothalamic neuropeptides (POMC, CART, NPY and AgRP). These in turn could relate to amino acid sensing systems present in the same area, related to BCAA and glutamine metabolism, as well as mechanistic target of rapamycin (mTOR), taste receptors and general control non-derepressible 2 (GCN2) kinase signalling. The treatment with valine did not affect amino acid sensing parameters in hypothalamus. These responses are comparable to those characterized in mammals. However, clear differences arise when comparing rainbow trout and mammals, in particular with respect to the clear orexigenic effect of valine, which could relate to the finding that valine partially stimulated two amino acid sensing systems in telencephalon. Another novel result is the clear effect of leucine on telencephalon, in which amino acid sensing systems, but not neuropeptides, were activated as in hypothalamus