The effects of feeding indole, indole-3-carbinol and benzofuran (all at 5 mmole/kg body wt./day) on various hepatic microsomal and cytosolic enzyme activities involved in xenobiotic metabolism have been compared. Benzofuran was found to elevate the activities of many enzymes both in microsomes (e.g., aniline hydroxylase, 7-ethoxycoumarin O-deethylase, p-nitrophenol UDPGA-transferase and epoxide hydrolase) and in cytosol (e.g., glutathione reductase, glutathione S-transferase, NADH:quinone reductase and UDP-glucose dehydrogenase). The structures of indole and indole-3-carbinol are similar to benzofuran except for the substitution of nitrogen with oxygen atom within the furan ring. Results showed that the activities of UDPGA-transferase and NADH:quinone reductase were not elevated by these indole compounds. While the chemical structure of these two indole compounds are identical except for the presence of the carbinol (methanol) group in indole-3-carbinol, there were marked differences in the types and activities of microsomal enzymes that were enhanced. Among the microsomal enzyme activities determined, indole elevated only the NADPH:cytochrome c reductase, while indole-3-carbinol increased several mixed function oxidase and particularly the epoxide hydrolase activities. Based on the chemical structures of tested compounds and the observed results, possible explanations for the mechanisms involved in elevating epoxide hydrolase activity by benzofuran and indole-3-carbinol are discussed.