OBJECTIVES: This study was conducted to provide accurate exposure evaluation of workers in a biologicallymonitored state who were simultaneously exposed to benzene and toluene. For the purpose of this study, an animal experiment was conducted. METHODS: The following concentrations of solvents were administered orally to Sprague-Dawley rats : benzene at 2.26 mg/kg body weight (equivalent concentration to the 2.5 ppm, Threshold Limit Value-ShortTerm Exposure Limit, in the USA) and 9.02 mg/kg body weight (equivalent concentration to the 10 ppm, Threshold Limit Value-TimeWeighted Average in Korea), simultaneously with toluene at 106.42 mg/kg body weight (equivalent concentration to the 100 ppm, Threshold Limit Value-TimeWeighted RESULTS: The following results were obtained from the analysis of reading taken at 3hour intervals of S-phenylmercapturic acid (SPMA) and phenol concentration in urine metabolites of benzene after oral administration for 30 hours. 1. The concentrations of phenol and SPMA in urine were markedly decreased in the initial phase of the mixed group (both benzene and toluene administered group) as compared with the benzeneonly administered group, and the concentrations were slightly elevated. 2. The total excreted amounts of phenol and SPMA in urine decreased in the mixed group compared with the benzene-only group, and these decreases were more remarkable at the benzene administration concentration of 9.02 mg/kg than at 2.26 mg/kg. 3. The urinary excretions of phenol and SPMA were delayed in the case of the mixed group, and the extent of the delay was dependent on the amount of benzene administrat CONCLUSIONS: Benzene metabolism was suppressed by toluene, and hence the excretion of phenol and SPMA as urinary metabolites of benzene was delayed. This result will have applications in the interpretation of results from future biological monitoring of workers exposed to mixed solvents. We should not underestimate the importance of carefully interpreting the results of biological monitoring data when workers are exposed to mixed solvents. We should not underestimate the importance of carefully interpreting the results of biological monitoring data when workers are exposed to mixed benzene and toluene.