The purpose of this study is to investigate differences in excretion of toluene and m-xylene. After intraperitoneal injection of toluene or m-xylene (5 mmole per kg body weight) to Wistar female rats, urine and expired air were collected regularly. The concentrations of toluene and m-xylene in expired air were determined spectrophotometrically. Their urinary metabolites hippuric acid and m-methylhippuric acid were measured by high-performance liquid chromatography. The results were: 1. The excretion rates in expired air for 10 hours after the administration of toluene and m-xylene were 37.7 % and 13.1 % respectively. After the injection of toluene, the excretion rate per hour of the vapor in expired air increased rapidly, and reached a maximum (6.1 % ) in 2 hours, then decreased in a half-life of 4.87 hours. The maximum excretion rate of m-xylene per hour was 1.6 % in 3 or 4 hours; thereafter, the excretion rate decreased more slowly than in the case of toluene. 2. The total excretion rate in urine for 48 hours after administration was 29.0 % for toluene and 43.5 % for m-xylene. Urinary excretion of m-xylene is apparently higher than that of toluene, in contrast to expiratory excretion. Following the administration of toluene, about 14 % had been eliminated as hippuric acid in the 0-10 and 10-24 hours' urine. The concentration of hippuric acid had returned to normal in the 24-48 hours' urine. m-Methylhippuric acid in the 0-10 hours' urine was 13.7 % of m-xylene; in the 10-24 hours' urine it was 22.3 % ; moreover, 7.5 % was found even in the 24-48 hours' urine. From these results, it is estimated that the velocity of urinary excretion of m-xylene is less than that of toluene. 3. The total excretion rate of toluene in urine and expired air was 51.9 % and that of m-xylene was 26.9 % , for 10 hours after the administration. Furthermore, the excretion rate of both 10 hours' expired air and 48 hours' urine, was 66.7 % for toluene and 56.6 % for m-xylene. 4. The ratio of 48 hours' urinary excretion to 10 hours' expiratory excretion was 0.77 for toluene and 3.58 for m-xylene. These results indicate that toluene was excreted more in expired air than in urine whereas m-xylene was mainly excreted in urine.