Weanling (22-day-old) rats fed a low protein (8% casein) diet consumed the same amount of energy as controls (22% casein diet), but intake corrected for body size (kJ/kg^0.75) was increased in the former group. Weight gain and the efficiency of gain (g gain/MJ) were markedly reduced in low protein fed rats. Resting oxygen consumption (VO₂) was elevated by 15% in the low protein group but this difference was completely abolished by β-adrenergic blockade with propranolol. Interscapular brown adipose tissue (BAT) mass, protein content, mitochondrial yield and GDP binding were increased in low protein fed rats but mitochondrial α-glycophosphate shuttle activity of BAT was unaltered, although shuttle activity was elevated in liver mitochondria. Plasma triiodothyronine levels were increased by 64% in the low protein group, whereas insulin levels were markedly reduced in spite of normal blood glucose levels. Resting VO₂ and BAT mass were also increased in older (55-day-old) rats fed the low protein diet, but the changes were smaller than in weanling rats. These data suggest that the decreased metabolic efficiency seen in rats fed protein deficient diets involves sympathetic activation of BAT, and is therefore similar to the thermogenic responses seen in cold adapted and cafeteria-fed animals.