Proinflammatory cytokines play an important role in the depression of cytochrome P450 (CYP450)-dependent drug metabolism in mammals during inflammation and infection. Although much has been learned concerning the effects and mechanisms of cytokine-mediated suppression of CYP450, there is limited knowledge about how cytokines affect UDP glucuronosyl transferases (UDPGT). The aim of the present study was to investigate the effects and dose dependency of recombinant human proinflammatory cytokines on both CYP450- and UDPGT-dependent enzyme activities in primary cultures of pig hepatocytes. A possible role of nitric oxide in cytokine-induced suppression of enzyme activities was studied by incubating hepatocytes in the presence ofN^G-nitro--arginine (L-NAME), a competitive inhibitor of nitric oxide (NO) biosynthesis. Incubation of hepatocytes with interleukin-1α (IL-1α), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) decreased both oxidation and glucuronidation activities dose dependently, in which the effects on glucuronidation activities were even more pronounced. IL-6 differed from IL-1α and TNF-α by inhibiting CYP450 and UDPGT more effectively after 24 hr of incubation, whereas the inhibition by IL-1α and TNF-α was more pronounced after 12 hr. Only at a concentration of 500 U/ml did interferon-γ (IFN-γ) inhibit CYP450 and UDPGT. The inhibition of CYP450 enzyme activities by cytokines was probably not due to the production of NO, because L-NAME totally blocked NO production but had no effect on the cytokine-induced suppression of CYP450 enzyme activities. However, there might be a role for NO in the decrease of glucuronidation by cytokines, as L-NAME slightly though significantly prevented the inhibition of glucuronidation.