Previous results from several laboratories have demonstrated that tumor necrosis factor-alpha (TNFalpha) depressed cytochrome P-450 (P-450)-dependent drug metabolism in vivo. However, there is some debate whether the action of TNFalpha is mediated by its direct effects on hepatocytes, or is indirectly mediated through the release of other mediators like IL-1 from macrophages. In the present studies, we investigated the effects of TNFalpha on P-450-dependent drug metabolizing enzyme as measured by 7-ethoxyresorufin O-deethylase (EROD) activity.