Male Sprague-Dawley rats were fed on purified diets supplemented with 50-500 ppm indole-3-carbinol (I3C), a compound present in cruciferous vegetables, or with 25% Brussels sprouts (Brassica oleracea) for 10 days after a 1-wk equilibration on a purified diet. Cytosolic and microsomal fractions were prepared from liver and intestinal mucosae. Intestinal aryl hydrocarbon hydroxylase (AHH) activity was increased significantly (P less than 0.05) over basal levels by I3C at 50 ppm (a 6.1-fold increase), at 125 ppm (11.8-fold), at 250 ppm (14.1-fold) and at 500 ppm (20.2-fold) and by 25% Brussels sprouts (3.6-fold). Intestinal ethoxycoumarin O-deethylase (ECD) activity was also significantly increased by I3C, the increases being 4.6-, 8.7-, 9.3- and 11.2-fold with 50, 125, 250 and 500 ppm I3C, respectively, and 3.2-fold with the sprouts diet. Hepatic AHH and ECD were not increased significantly by any of these dietary treatments. Hepatic and intestinal glutathione S-transferase (GST) activities were increased (P less than 0.05) 1.9- and 1.6-fold, respectively, by the sprouts diet but were not significantly affected even by 500 ppm I3C. Microsomal epoxide hydrolase (EH) activity of the small intestine was increased 2.0-fold by the sprouts diet but was unaffected by I3C. Hepatic cytochrome P-450 was increased 1.3-fold by the sprouts diet although I3C at 500 ppm only produced a 1.1-fold increase. A no-effect-level for I3C on intestinal monooxygenase induction was estimated to be between 16 and 25 ppm, supporting the contention that I3C can account for much of the monooxygenase induction observed when experimental animals are fed diets high in cruciferous vegetables. The results also indicate that Brassica oleracea contains other compounds which are responsible for the induction of GST and EH activities.