Cytochrome P450 (CYP) is a diverse superfamily of hemoproteins found in a large number of organisms.Most cytochrome P450 enzymes have monooxygenase activity and are involved in detoxification and hormone and lipid metabolism.In drug metabolism,the chemical modification or degradation of chemicals of exogenous or endogenous source,cytochrome P450 is probably the most important element of oxidative metabolism.It has been known for three decades that the expression of cytochrome P450 gene family members is regulated by the biological clock.In addition,hepatic P450 monooxygenases metabolize melatonin,the pineal hormone whose expression is controlled by the biological clock and,in turn,resets the biological clock.However,it remained the subject of intense scientific research to better understand how the expression of cytochrome P450 enzymes is regulated at the molecular level.The present understanding of the mechanisms of induction of cytochrome P450 enzymes and their regulation has made considerable progress during the last few years.Recently,it has been elucidated how the biological clock regulates the expression and activity of cytochrome P450 enzymes to affect pharmacokinetics,detoxification,and hormone and lipid metabolism.In addition,melatonin metabolism and cytochrome P450 enzyme activity have been found to affect circadian rhythms in mammals.These findings demonstrate the intricate relationships between the biological clock and cytochrome P450 enzymes and the importance of further study of chronotherapy and chronotoxicity.