This study was undertaken to investigate the effects of organic solvents on xenobiotic metabollzing enzyme system in vivo by meaas of experimental conditions i.e. (1) single group which was treated by benzene (B), toluene (T) and xylene (X), respectively, (2) combination group which was treated by mixture of benzene+toluene (BT), benzene+xylene (BX), and toluene+xylene (TX), respectively, (3) mixture group which was treated by benzene+ toluene+xylene mixture (M), and to interpreat the interaction between the organic solvents metabolizing enzymes. 1. The contents of cytochrome P-450 in liver microsomes were increased (p < 0.01) in organic solvents treated groups, and the contents of cytochrome P-450 were increased by following order of B < T < M < BT=BX < X < TX. 2. The activity of cytochrome P-450 dependent AHHase was significantly higher in organic solvents treated groups than in control group (p < 0.01), and the activity of AHHase was increased by following order of B < T < BT=BX=TX=xylene < M. 3. The activity of NADPH P-450 reductase was significantly higher in organic solvents treated groups than in control group (p < 0.01), and the order of M < combinated group < X < T <B, however, the activity of NADH b5 reductase was the reverse order of NADPH P-450 reductase activity. 4. The activity of superoxide dismutase (SOD), and generation of hydroxyl radical and lipid peroxide were significantly higher in organic solvents treated groups than in control group (p < 0.01), and the order of M < T=X=combinated group (BT=BX=TX) < B. These results suggested that the first step of biotransformation pathway of organic solvents(benzene, toluene and xylene) may be proceeded by a different metabolic reaction. Because benzene was metabolized by radical reaction through cytochrome P-450 dependent radical intermediates, meanwhile toluene and xylene were hydroxylated by cytochrome P-450 dependent AHHase.