Abstract: Many carcinogens must be first transformed into electrophilic ultimate carcinogens via metabolic activation in liver microsomes before covalent binding to nucleophilic center of DNA. SY-640 is a synthetic compound with hepatoprotective activity. Results of the present study indicate that the covalent binding of ³H-benzo(a)pyrine to mouse hepatocyte nuclear DNA in vitro and in vivo was markedly inhibited by SY-640. Further studies found that the liver microsomal cytochrome P-450 content and aminopyrine demethylase activity were significantly increased in mice treated with SY-640(150 mg·kg^-1 po) once daily for three days, while the hepatic microsomal aminopyrine demethylase activity was obviously inhibited two hours after oral administration of SY-640 150 mg·kg^-1 in mice. The aminopyrine demethylase activity of liver microsomes from normal, PB- and 3-MC-treated mice was also significantly inhibited by the addition of SY-640 in vitro. When SY-640 was incubated with NADPHreduced mouse liver microsomes, a metabolic-intermediate(MI) complex at 457 nm was formed. The effects of SY-640 on cytochrome P-450 and its formation of MI complex with cytochrome P-450 may partially explain why SY-640 could inhibit covalentbinding of BP to mouse hepatocyte DNA in vitro.