In an attempt to clarify the pathways of metabolic activation, the emphasis has been on the CYP monooxygenase system. However, a report of concomitant inhibition and induction of the 2 major monooxygenase systems, CYP monooxygenase and flavin-containing monooxygenases, by a single compound (33) suggests another potential layer of complexity in response to the mixture of compounds in a typical diet. Flavin-containing monooxygenases, not thought to be modulated by xenobiotics, were recently shown in rats to be inhibited by indole-3-carbinol (I3C), the precursor of which is found in cruciferous vegetables, coincident with induction of CYP1A1 (33). Whether a similar phenomenon occurs in humans and how these types of effects may influence metabolism of the variety of xenobiotics to which humans are exposed are questions that also need to be explored.