Tissue distribution, metabolism and excretion of 14C-labeled toluene were investigated after a single intraperitoneal administration (290 microgram/kg) of the compound into mice. The highest radioactivity was detected in the adipose tissue, followed in descending order by the kidney, liver and lung. The lowest radioactivity was retained in brain tissue and the brain/blood concentration ratio was about 0.4 throughout. Radioactivity in the blood declined exponentially and the biological half-life was estimated to be 25 min. Radioactive materials detected at as early as 8 min in the kidney (78%) and liver (64%) proved to be non-volatile metabolites. On the contrary, 70% of radioactive materials in the brain and near 100% in the adipose tissue were found to be a volatie compound (probably unchanged toluene). The cumulative urinary excretion of radio-activity was 26.4% of the dose at 30 min and 73.8% at 18 hr, whereas the pulmonary or fecal excretion was negligibly small. Radioactive materials excreted in the urine were identified by paper and the thin-layer chromatography as hippuric acid (59%) and benzoylglucuronic acid (41%). These results show that toluene is metabolized rapidly and is excreted mainly in the urine. The relative importance of glucuronide formation in detoxication mechanisms was noted.