To investigate the roles of the autonomic nervous system in the thermic effects of protein and carbohydrates in rats, we determined the urinary excretion of catecholamines and the resting oxygen consumption by means of HPLC-fluorometry and open-circuit respirometry, respectively. Protein administration significantly increased the urinary excretion of norepinephrine and epinephrine over those on water administration. The thermic effect of protein was 16.6% of the basal metabolic rate and was inhibited by phentolamine, prazosin, or atropine, but not by propranolol. These results suggest that the sympathetic nervous system via alpha 1-adrenoceptors and the parasympathetic nervous system are involved in the thermic effect of protein. The administration of carbohydrates such as glucose, sucrose, and fructose significantly enhanced the urinary excretion of norepinephrine, but only glucose administration increased the urinary excretion of epinephrine. The thermic effects of carbohydrates were 8-9% of the basal metabolic rate and were inhibited by propranolol, but not by phentolamine or atropine. These findings suggest that the sympathetic nervous system via beta-adrenoceptors, but not the parasympathetic nervous system, contributes to the thermic effect of carbohydrates. Thus, we conclude that the autonomic nervous system is involved in the thermic effects of protein and carbohydrates by different mechanisms.