It is well known that the peripheral peptide YY (PYY)-central neuropeptide Y (NPY) Y2 receptor axis plays an important role in promoting negative energy balance regulation. Both the hypothalamus and medulla oblongata express a high level of Y2 receptors; however the functional role of this receptor in chronic high-fat diet-induced obesity has not been fully examined. Using quantitative autoradiography, this study measured binding densities of total [¹²⁵I]-PYY and Y2 receptors in the hypothalamus and medulla of chronic high-fat diet-induced obese (DIO), obese resistant (DR) and low-fat fed (LF) mice. Plasma PYY was also measured using radioimmunoassay after 22 weeks of dietary intervention. The results revealed that body weight gain was significantly higher in the obese mice compared to the lean mice. Furthermore, PYY and NPY Y2 receptor binding densities in the medulla of the obese mice were significantly higher compared to the lean mice while the level of plasma PYY was significantly lower in the DIO mice than the LF mice. In conclusion, this study showed that the DIO mice had low plasma PYY which may have caused a compensatory upregulation of PYY and Y2 receptor densities in the medulla. A low level response of PYY-medullary regulation to positive energy balance may have contributed to the development of high-fat diet-induced obesity in DIO mice; conversely, a normal response of this regulatory axis in the DR mice may have contributed to the maintenance of body weight whilst on a high-fat diet.