The mode of action of capsaicin on energy metabolism was investigated in rats. The oxygen consumption was higher when capsaicin (6.0 mg/kg) was intraperitoneally injected than when it was not injected. The respiratory quotient (R.Q.) increased and then decreased after the administration of capsaicin. The levels of serum glucose and immunoreactive insulin rapidly increased after the administration of capsaicin. Also, liver glycogen rapidly decreased, in contrast to the serum glucose concentration which rapidly increased. The serum-free fatty acid level gradually increased after the administration of capsaicin. These alterations in energy metabolism on the administration of capsaicin were similar to those in the metabolism of epinephrine, and were specifically inhibited by various beta-adrenergic blockers. On the other hand, the alterations were not affected by pretreatment with alpha-adrenergic or ganglion blockers. These results suggest that the mode of action of capsaicin on the enhancement of energy metabolism in rats comprises a direct (as an agonist) and/or an indirect (via catecholamine) beta-adrenergic action. Therefore, it was speculated that the adrenergic action of capsaicin resulted, at least in part, in a decrease in the perirenal adipose tissue weight and serum triglyceride concentration in rats fed a high fat diet supplemented with capsaicin (T. Kawada et al., J Nutr 116:1272-1278, 1986).