Administration of metformin is known to reduce both body weight and food intake. While the hypothalamus is recognized as a critical regulator of energy balance and body weight, there is currently no evidence for an effect of metformin in the hypothalamus. Therefore, we sought to determine the central action of metformin on energy balance and body weight as well as its potential involvement with key hypothalamic energy sensors, including AMP-activated protein kinase (AMPK) and S6 kinase (S6K). We used meal pattern analysis and a conditioned taste aversion (CTA) test, and measured energy expenditure in C56BL/6 mice administered metformin (0, 7.5, 15, or 30 μg) into the third ventricle (I3V). Furthermore, we I3V-administered either control or metformin (30 μg) and compared the phosphorylation of AMPK and S6K in the mouse mediobasal hypothalamus. Compared to the control, I3V administration of metformin decreased body weight and food intake in a dose-dependent manner and did not result in CTA. Furthermore, the reduction in food intake induced by I3V administration of metformin was accomplished by decreases in both nocturnal meal size and number. Compared to the control, I3V administration of metformin significantly increased phosphorylation of S6K at Thr³⁸⁹ and AMPK at Ser^485/491 in the mediobasal hypothalamus, while AMPK phosphorylation at Thr¹⁷² was not significantly altered. Moreover, I3V rapamycin pretreatment restored the metformin-induced anorexia and weight loss. These results suggest that the reduction in food intake induced by the central administration of metformin in the mice may be mediated by activation of S6K pathway.