Diet-induced obesity (DIO) developed in 1-mo-old male Sprague-Dawley rats over an 8-wk period on a relatively high-fat (16%) high-calorie (4.6 kcal/g) diet (DIO diet). Percent carcass lipid (56%) and body weight gain (15%) were greater, whereas food intake was decreased over the first 3-5 wk in DIO diet-compared with chow-fed controls. Overall, 8-wk body weight gain (15%), percent carcass lipid (26%), and feed efficiency (15%) were greater, but food intake was not increased. Norepinephrine (NE) turnover rate, indicative of organ sympathetic activity, increased in interscapular brown adipose tissue (IBAT; 57-218%), heart (21-44%), and pancreas (25%) during the first 3 wk and remained elevated for the entire 8 wk. IBAT weight (51%) and in vitro lipolytic capacity (68%) increased by 1 wk and brown adipocyte size (43%) by 3 wk; IBAT thermogenic capacity (maximal NE-stimulated in vitro O2 consumption) increased by 5 wk (39%). Plasma insulin levels were similar in both diet groups over the entire 8-wk period. Why DIO diet-fed rats had increased metabolic efficiency is unknown, but activation of IBAT metabolism and thermogenesis failed to prevent the development of DIO.