The effects of capsaicin, a pungent principle of hot red pepper, on body fat gain, balance serum lipid values were investigated in rats fed a high-fat(30%) diet. Administration of capsaicin by dietary administration caused a complete cessation of increased in body weight and fat gain induced by the high-fat diet. However, energy intake and body protein gain were not affected by capsaicin. Therefore, the suppression of body fat gain by capsaicn was believed due to an increased in energy expenditure. Simultaneous administration of capsaicin and a $eta$-adrenergic blocker, propranolo, resulted in the inhibition of changes in body fat gain by capsaicin without remained unchanged, indicating an increase in the number of mitochondria in brown adipose tissue. Therefore, it appears that capsaicin possesses potent body fat suppressive effects mediated by $eta$-adrenergic stimulation in which brown adipose tissue may be involved. On the other hand, capsaicin had no effects on serum triglyceride, total cholesterol or HDL-cholesterol levels. These results are in contrast to those reported by other investigators. Perhaps expression of the effects of capsaicin on plasma lipids is a rather complicated process, dependent on the type of diet administered, fat content of the diet, period and route of capsaicin administration, and species and strain of animals used.