The modification of CYP2E1 activity is of considerable interest because of its role in the metabolic activation of a variety of toxic chemicals. In the present studies, the time-course of changes in human CYP2E1 activities was determined after treatment with ethanol, glycerol, 4-methylpyrazole or isoniazid using intact HepG2 cells transfected by human CYP2E1. Hydroxylation of chlorzoxazone was chosen for the measurement of CYP2E1 activity. CYP2E1 protein levels were increased upon cultivation of cells in the presence of ethanol, glycerol, 4-methylpyrazole or isoniazid for 24 hr. After 24 hr cultivation, ethanol or glycerol increased CYP2E1 activities, whereas 4-methylpyrazole or isoniazid inhibited. This different effect of the chemical inducers on CYP2E1 activi-ties persisted to subsequent 24 hr. Competitive inhibition study suggested that 4-methylpyrazole or isoniazid has stronger binding affinity to CYP2E1 than ethanol or glycerol. These results demonstrate that different binding affinity of the chemical inducers to the active site of CYP2E1 plays important role in determining real CYP2E1 activity in intact cells after treatment with the chemical inducers. Present study would be helpful in precise understanding of human CYP2E1-mediated toxicity.