To investigate the circadian variation in the bromobenzene metabolism, bromobenzene(400 mg/kg body weight) was intraperitoneally administered to the rats every other day for 6 days both in the night; 24:00 and the day; 12:00. Each group of animals was sacrificed at 8hr after last injection of bromobenzene. The contents of hepatic CYP were more increased in control rats of night phase than those of day phase but in case of bromobenzene treatment there were no differences in hepatic CYP between rats of the night phase and those of day phase and the injection of prednisolon inhibited the hepatic CYP content in rats. Furthermore, the decreasing rate of hepatic glutathione contents to the control was higher in rats of day phase than those of night phase by the bromobenzene treatment. And the hepatic glutathione S-transferase activities were increased both in control and bromobenzene treated rats of the night phase than those of day phase. On the other hand, liver weight per body weight(%), hepatic lipid peroxide content, serum levels of alanine aminotransferase were more increased both in bromobenzene-treated and control rats of the night phase than those in the day phase. These results indicate that the rats of night phase may induce more accelerated formation of bromobenzene 3,4-oxide from bromobezene than those of day phase in rats.