Inflammatory stimulation of the liver induces nitric oxide (NO) biosynthesis and suppression of detoxication. In this study the effect of NO biosynthesis on cytochrome P-450 (CYP) enzyme activity was investigated by comparing in vivo and in vitro assays. To establish liver inflammation, CD rats were injected with Corynebacterium parvum (C. parvum) suspension. After 5 days NO biosynthesis was highly induced as indicated by increased NO2- plus NO3- serum concentrations. At the same time the aminopyrine breath test (ABT), measuring CYP activity in vivo, was reduced to 42% and the in vitro assay of aminopyrine turnover was suppressed to 12% of NaCl- injected controls. When C. parvum-injected animals were treated with the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA), CYP activities significantly improved with an ABT of 76% and an in vitro aminopyrine turnover of 47% of controls. Neither C. parvum injections nor L-NMMA treatment resulted in a significant change of CYP protein concentrations. These data indicate that suppression of xenobiotic metabolism can be attenuated by inhibition of NO biosynthesis during an ongoing process of inflammation.