The aim of our study was to determine which microsomal cytochrome P450 isozyme(s) were responsible for the microsomal oxidation of indole to indoxyl, an important intermediate in the information of the uremic toxin indoxyl sulfate. Indole was incubated together with an NADPH — generating system and rat liver microsomes. Formation of indigo, an auto-oxidation product of indoxyl, was used to determine the indole-3-hydroxylation activity. Apparent K_m and V_max values of 0.85 mM and 1152 pmol min⁻¹ mg⁻¹ were calculated for the formation of indoxyl from indole using rat liver microsomes. The effects of various potential inducers and inhibitors on the metabolism of indole to indoxyl by rat liver microsomes were studied to elucidate the enzymes responsible for metabolism. Studies with general and isozyme-specific P450 inhibitors demostrated that P450 enzymes and not FMO are responsible for the formation of indoxyl. In the induction studies, rate of indoxyl formation in the microsomes from untreated vs induced rats correlated nearly exactly with the CYP2E1 activity (4-nitrophenol 2-hydroxylation). These results suggests that CYP2E1 is the major isoform for the microsomal oxidation of indole to indoxyl.