After an initial equilibration period of 7 days on a semi-synthetic basal diet, male Sprague-Dawley rats were fed for 2 wk on either the basal diet (controls), the basal diet containing 5% Schizandra chinensis or 25% Brussels sprouts, or on rat chow. One group of chow-fed rats was pretreated with 20 mg 3-methylcholanthrene (3-MC)/kg body weight, 24 hr before they were killed. Microsomal and cytosolic fractions were prepared from small intestine mucosa. Microsomes were assayed for cytochrome P-450, aryl hydrocarbon hydroxylase (AHH), ethoxycoumarin O-deethylase (ECD) and epoxide hydrolase (EH) activities, and for metabolism of benzo[a]pyrene (BaP), Cytosols were assayed for glutathione S-transferase (GST) activity. The largest increase in intestinal mixed-function oxidase activity over levels in the controls was seen in the 3-MC-treated group. However, EH and GST activities in these animals were not significantly increased. Increases in cytochrome P-450 levels and significant increases in AHH, ECD, EH and GST activities occurred in the rats fed Brussels sprouts. Rats in the S. chinensis group showed inhibition of AHH activity relative to controls, but increased activity of ECD, EH and GST. In the rats fed chow there were significant increases in the activities of all the enzymes assayed except GST. The percentage conversion of BaP to metabolites reflected the results of the AHH assay and the groups were ranked in the following order: 3-MC greater than Brussels sprouts greater than rat chow greater than basal diet greater than S. chinensis. The profile of BaP metabolites showed a larger proportion of the BaP diols and 3-hydroxybenzo[a]pyrene, and a smaller proportion of BaP-4,5-epoxide and the BaP quinones, for the Brussels sprouts- and S. chinensis-fed groups. The significance of these results is discussed in regard to the role of the small intestine as a mediator of toxicity induced by ingested chemicals.