Cytochrome P450 function was compared in rats maintained on cereal-based chow or a purified diet that simulates nutrient levels present in the chow. Ethoxycoumarin O-deethylase activity was increased in small intestine (15-fold), colon (2.5-fold) and liver (1.6-fold) by cereal chow. Ethoxyresorufin O-deethylase activity was also increased by cereal chow (75- and 2.5-fold in small intestine and liver, respectively). Cytochrome P450IAI, detected by Western blotting, was observed in intestinal microsomes isolated from chow-fed rats. These data underscore the importance of dietary considerations when evaluating studies of xenobiotics that undergo 'first pass' cytochrome P450-dependent metabolism in the small intestine.