The characteristics of regional brown (BAT) and white adipose tissue (WAT) growth and of thermogenesis following experimental overfeeding were studied in groups of male Sprague-Dawley rats fed lab chow or cafeteria diets for 8 weeks postweaning. Regional BAT and WAT growth was determined by dissection and weighing, and thermogenesis was characterized by measurements of resting and norepinephrine (NE)-stimulated oxygen consumption, of serum thyroid hormone concentrations, and of 24-hour urinary NE excretion levels. Cafeteria feeding resulted in a 113% increase in total BAT, with the most prominent increases in the interscapular, thoracic, and perirenal regions. Retroperitoneal, epididymal, and omental WAT were significantly greater in cafeteria than in chow-fed rats. Resting oxygen consumption of cafeteria-fed rads increased by 10% and NE excretion by 64% compared to chow-fed controls, while serum T3 concentrations were nearly doubled in the cafeteria-fed rats. The thermogenic response to NE injection in cafeteria-fed rats was 102% of their resting levels, compared to a 51% increase in the chow-fed controls. The results indicate that increased BAT growth occurs in all primary BAT depots following cafeteria-feeding in rats, and that the greater BAT mass is qualitatively proportional to their greater capacity for non-shivering thermogenesis. Also, the increased NE excretion and greater serum T3 concentration are consistent with increased sympathetic and thyroidal activity and may in part explain the thermogenic response to diet in the rat.