Treatment with inducers of cytochrome P450 2E1 such as ethanol, pyrazole or dimethyl sulfoxide (DMSO) increased the aniline hydroxylase activity in primary culture of rat hepatocytes. Large amounts of cytochrome P450 2E1 are present in the hepatocytes from freshly isolated. Culturing of hepatocytes caused a rapid disappearence of cytochrome P450 2E1 activity from the cells. However, the presence of 2\% DMSO in the medium prevented the loss of enzyme activity. In order to investigate whether cytochrome P450 2E1 expression is related to cellular signal transduction system, we used known protein kinase inhibitors (staurosporine, genistein, R24571) and activators (PMA, dibutyryl cAMP). But, most of them, except dibutyryl cAMP, had little effects on aniline hydroxylase activity. Dibutyryl cAMP decreased aniline hydroxylase activity both in the absence and presence of DMSO. Glucagon, hormone activating adenylate cyclase cascade in liver, also brought out the same result in rat hepatocyte cultures. These results support the hyopothesis that DMSO directly bind to cytochrome P450 2E1 and make it less sensitive to phosphorylation by cAMP-dependent protein kinase and subsequent degradation.