A model of dietary obesity was developed, in adult male rats, where duration of feeding of a high fat diet (HFD; 60 percent of calories as fat) influenced capacity to reverse the obese state following a reduction in dietary fat. After 17, and again after 30 weeks, groups of HFD fed rats were returned to ad libitum feeding of a low fat, chow diet (C; 14 percent fat). The rats switched from HFD to C after 17 weeks returned to control levels of body weight and body composition, but the rats switched from HFD to C after 30 weeks did not. This latter group remained heavier than controls without ingesting more energy, indicating an increased energy efficiency and a reduction in energy requirements. These results suggest that energy balance during development of dietary obesity may be different from energy balance in long-standing dietary obesity. In addition to demonstrating that duration (and/or severity) of dietary obesity is an important variable in the potential reversal of the obese state, the results provide further evidence that persistent obesity is not maintained solely by an elevated energy intake. A corollary to these findings is that treatment of dietary obesity may be more effective if begun early.