In order to estimate the influence of phenobarbital on the metabolism of m-, p-xylene, female Wistar rats were pretreated with intraperitoneal injection of sodium phenobarbital (75 mg per kg body weight) for 3 days (PB group). On the 4th day, 4.667 mmole per kg of m-xylene or p-xylene was injected intraperitoneally and urine samples were collected at 4 hour intervals thereafter. The urinary content of m-methylhippuric acid and p-methylhippuric acid was determined by high-performance liquid chromatography. Urinary excretion of m-methylhippuric acid or p-methylhippuric acid in the PB group was significantly higher than in the control group (pretreated with 2 ml per kg of normal saline instead of phenobarbital) at the 0-4 hour and 4-8 hour intervals (p<0.01). This suggests that oxidation of methyl groups of m- and p-xylenes can be performed by the microsomal drug metabolizing enzyme in liver. There was no significant difference between the PB and control groups in the amount of m-methylhippuric acid and p-methylhippuric acid excreted during the first 48 hours after administration.