Drug-metabolizing system which has the important role in drug metabolism is localized in smooth endoplasmic reticulum of hepatocytes and is composed of NADPH, NADPH-cytochrome $P_{450}$ reductase, cytochrome $P_{450}$ and others. It is well known that the enzyme system is induced by phenobarbital and methylcholanthrene. Lipid peroxidation is reaction of oxidative deterioration of polyunsaturated lipids. Formation of lipid peroxides in liver microsome has been found to produce degradation of phospholipid, which are major components of microsomal membrane. The relationship between the formation of lipid oxides and the activities of drug-metabolizing enzyme in the liver of rats was reported by several investigators. In this study the effect of riboflavin tetrabutylate, an antioxidant on lipid peroxidation, specially the relationship between lipid peroxidation and drug-metabolizing enzyme system was investigated. In addition the effect of vitamin A derivatives, such as retinoic acid and retinoid on the enzyme was also observed. Results are summarized as followings. 1) The pretretment with riboflavin tetrabutylate inhibited completely the lengthened sleeping time due to $CCl_{4}$ treatment. 2) The increase of TBA value was prevented by the pretreatment with riboflavin tetrabutylate. 3) The pretreatment with riboflavin tetrabutylate also prevented the decrease of drug-metabolizing enzyme caused by $CCl_{4}$. 4) Both retinoic acid and retinoid remarkably decreased the activity of aminopyrine demethylase. Pretreatment of riboflavin tetrabutylate, however, prevented inhibitory effect of retinoic acid on the enzyme activity.