To evaluate an effect of aging on the xylene toxicity, 50% m-xylene in olive oil (0.25 ml/100 g body wt.) was administered to 5 week and 12 week-old rats one times intraperitoneally and sacrificed at 24 hrs afterwards. The increasing rate of urinary m-methylhippuric acid concentration was higher in 12 week-old rats than 5 week-old rats by the treatment of m-xylene. On the liver function findings, i.e., liver weigh/body weight (%), serum levels of ALT activity and hepatic malondialdehyde content, 12 week-old rats showed more severe liver injury than 5 weeks those in xylene-treated rats. And the hepatic cytochrome P-450 contents was higher in 12 weeks rats than those of in 5 week-old rats, but the increasing rate of that was lower in 12 week-old rats. Hepatic alcohol dehydrogenase activity was also higher in 12 week-old rats than in 5 week-old rats whereas the increasing rate of that was higher in 5 week-old than those in 12 week-old rats by the xylene treatment. Furthermore, the hepatic aldehyde dehydrogenase activities were no differences between the 5 and 12 week-old rats both in the control and xylene-treated group. In conclusion, age may influences upon the hepatotoxicity with xylene and it may be responsible for xylene metabolism in rats.