Influences of capsaicin on the activities of cytochrome P45O of liver cell were studied in rats. Rats were provided food and water ad libitum and capsaicin and methylcellulose were gavaged for 6 days. Body weight gain and liver weight/body weight ratio, microsomal protein content and serum HDL- cholesterol content, the activity of cytochrome P450 and erythromycin demethylase, the activities of ethoxyresorufin and pentoxyresorufin O- dealkylase were determined. Capsaicin increased body weight gain but showed no significant changes on liver weight as compared with control group. Capsaicin increased the microsomal protein significantly but decreased the serum HDL- cholesterol. Capsaicin decreased the microsomal cytochrome P4SO significantly and did not show any influences on erythromycin demethylase ( cytochrome P45O III A ). Capsaicin increased the activity of pentoxyresorufin O- dealkylase ( cytochrome P45O II B) and decreased the activity of ethoxyresorufin O-dealkylase ( cytochrome P45O I A). It might be concluded that capsaicin reduced the microsomal cytochrome P45O and induced the CYP III and inhibited the CYP I A. It also might be concluded that capsaicin had no influence on CYP III A and decreased serum HDL- cholesterol. In these results capsaicin can not be used as an anti- atherosclerotic agent by increasing the CYP III A and HDL- cholesterol but it is considered that the more precise study on these theme is necessary.