Objectives: Dieting often fails because weight loss triggers strong counter-regulatory biological responses such as increased hunger and hypometabolism that are thought to be critically dependent on the master fuel sensor in the mediobasal hypothalamus (MBH). Because prolonged starvation has been shown to increase AgRP and NPY, the expression level of these two orexigenic genes has been taken as an experimental readout for the presence or absence of hunger. Roux-en-Y gastric bypass (RYGB) surgery leads to a significant weight loss without inducing the associated hunger, indicating possible changes in hypothalamic neuropeptides and/or signaling. Our goal was to assess key genes in the MBH involved in regulating body weight, appetite, and inflammation/oxidative stress after RYGB surgery in mice. Methods: Obese mice on a high-fat diet were subjected to either sham or RYGB surgery, or caloric restriction to match the weight of RYGB group. Chow-fed mice without surgery served as an additional control group. After 2 or 12 weeks post-surgery, hypothalamic genes were analyzed by real-time qPCR. Results: During the rapid weight loss phase at 2 weeks after RYGB surgery, hypothalamic AgRP and NPY gene expression was not increased compared to mice with sham surgery, indicating that the mice are not hungry. In contrast, the same weight loss induced by caloric restriction promptly triggered increased AgRP and NPY expression. This differential effect of RYGB and caloric restriction was no longer observed during the weight-maintenance phase at 12 weeks after surgery. A similar differential effect was observed for ObRb, but not for POMC and CART expression. Furthermore, RAGE and IBA-1, two markers for inflammation/oxidative stress, were significantly suppressed after RYGB compared to caloric restriction at 2 weeks post-surgery. Conclusions: These findings suggest that RYGB prevents the biologically adaptive hunger response triggered by undernutrition and weight loss, and suppresses weight loss-induced hypothalamic inflammation markers.