The X-linked orphan receptor, GPR50 shares 45% homology with the melatonin receptors, yet its ligand and physiological function remain unknown. Here we report that mice lacking functional GPR50 through insertion of a lacZ gene into the coding sequence of GPR50, exhibit an altered metabolic phenotype. GPR50 knockout mice maintained on normal chow (NC) exhibit lower body weight than age-matched wild type littermates by ten weeks of age. Further, knockout mice were partially resistant to diet-induced obesity. When placed on high-energy diet (HED) for five weeks, knockout mice consumed significantly more food per unit body weight, yet exhibited an attenuated weight gain and reduced body fat content compared with wild type mice. Wheel-running activity records revealed that although GPR50 knockout mice showed no alteration of circadian period, the overall levels of activity were significantly increased over wild types in both nocturnal and diurnal phases. In line with this, basal metabolic rate (O₂ consumption, CO₂ production, and respiratory quotient) was found to be elevated in knockout mice. Using in situ hybridisation (wild type mice) and -galactosidase activity (from LacZ insertion element in knockout mice), brain expression of GPR50 was found to be restricted to the ependymal layer of the third ventricle and dorsomedial nucleus of the hypothalamus. GPR50 expression was highly responsive to energy status, showing a significantly reduced expression following both fasting and 5 weeks of HED. These data implicate GPR50 as an important regulator of energy metabolism.