To evaluate an effect of growth periods on the bromobenzene-induced liver damage, bromobenzene was administrated to 5-week-old rats and 10-week-old rats pretreated with bromobenzene 5 times every other day for 10 days and then the animals were sacrificed. The results were obtained as follows; The increasing rate of serum levels of alanine aminotransferase, xanthine oxidase activity, hepatic lipid peroxide contents, liver weight per body weight (％) and decreasing rate of hepatic contents of protein to each control group were higher in 10-week-old rats than 5-week-old rats by the pretreatment of bromobenzene. According to the above results, 10-week-old rats indicated more severe liver injury than 5-week-old those in case of bromobenzene pretreatment. On the other hand, hepatic aniline hydroxylase activity was more increased in 10-week-old rats than 5-week-old rats both in control and bromobnezene pretreated rats where as the reverse in hepatic glutathione S-transferase. In case of hepatic GSH determination at the intervals of 2, 4, 8, 24 hours throughout 24 hr after administration of single dose of bromobenzene to 5-week-old and 10-week-old rats both in control and bromobnezene pretreated, the rate of GSH utilization was lower in 10-week-old rats than 5-week-old rats. In conclusion, from the above experimental, it is deduce that the 10-week-old rats showed more severe liver injury than 5-week-old rats by the bromobenzene treatment because the disposal ability of bromobenzene in liver was lower in 10-week-old rats than 5-week-old rats.