The functional state of interscapular brown adipose tissue (IBAT) was examined in rats fed for 20-30 d a high protein, carbohydrate-free diet [70% (wt/wt) protein, 8% fat] or a balanced diet (66% carbohydrate, 17% protein, 8% fat). In rats fed the high protein diet, body weight did not differ from that of control rats, but relative IBAT weight (grams per 100 g body wt) and lipid concentration (per gram of tissue) were 37% and 14% lower, respectively. In vivo rates of lipogenesis in IBAT, epididymal and retroperitoneal adipose tissue of rats fed the high protein diet were 20, 30 and 40%, respectively, of control values. Mitochondrial protein and cytochrome oxidase activity per total IBAT were significantly lower in rats fed the high protein diet than in controls; GDP binding was lower even when expressed per total tissue or per milligram of mitochondrial protein. The increase of IBAT temperature following norepinephrine infusion was significantly smaller than in controls. It is suggested that the decrease in IBAT capacity in the rats fed the high protein diet was due, at least in part, to a sustained reduction of sympathetic activity.