The regulation of feed intake in fish is dependent upon different neuroendocrine and metabolic mechanisms including amino acid sensing occurring in the gastrointestinal tract. In previous studies, we demonstrated the impact of specific amino acids alone on such mechanisms and their impact on feeding regulation. However, there is no information regarding the impact of diets with different protein levels or sources on those mechanisms. Therefore, in this study, we evaluated the response of amino acid sensing systems along the gastrointestinal tract (GIT) of rainbow trout (Oncorhynchus mykiss) fed with two different diets differing in the amount of fishmeal and vegetable protein: control diet with 20% fishmeal and 10% soy protein concentrate (NF/LV), and the other with 10% fishmeal and 20% soy protein concentrate (LF/HV). Fish were fed for 4 weeks and feed intake was registered daily with no significant differences found. At the end of the feed experimental trial, we collected samples of different areas of the GIT (stomach, anterior intestine and posterior intestine) and hypothalamus at different times: time 0, after 48h fasted; and times 1, 2 and 3, at 1h, 4h and 24h respectively after refeed. Gastrointestinal levels of mRNAs encoding amino acids receptors and transporters as well as mRNA and protein levels of key gut hormones (GHRL, CCK, PYY, GLP-1) were measured by RT-qPCR and Western Blot, respectively. Moreover, we assessed neuropeptide expression (NPY, AgRP, CART and POMC) in the hypothalamus and enzyme activity of pepsin in the stomach and trypsin and quimiotrypsin in the anterior intestine.