1 To study the effect of interferon on hepatic drug metabolism in chronic hepatitis C, we examined nine patients before and at the end of 6 months of interferon treatment. 2 Routine liver function was determined together with the salivary clearance of antipyrine and the 48 h urinary excretion of the main metabolites of antipyrine: 4-hydroxyantipyrine 3-hydroxymethylantipyrine and norantipyrine before and after 6 months of interferon treatment. 3 Liver pathology, routine liver function, and antipyrine metabolism remained unchanged after patients were treated for 6 months with interferon for a histologically advanced but clinically compensated chronic hepatitis C. ,