The effects of levels (5 or 30%) and types (saturated or unsaturated) of dietary fat on the procarcinogen-metabolizing cytochrome P 450 isoenzymes of duodenal, proximal, and distal ileal, colonic, and hepatic microsomes were studied.No significant effect of the diet were observed on liver cytochrome P 450, cytochrome b5, and cytochrome P 450 reductase.However, microsomes isolated from the ileum and colon expressed higher NADPH-cytochrome c (P 450) reductase, cytochrome P 450, and cytochrome b5 activities in response to either 5 or 30% unsaturated diets.There was a little evidence for dose or type of fat-associated effects on the expression of cytochrome P 450 isoenzymes on aniline or ethoxyresorufin deethylase substrates.Hepatic microsomes of rats fed the unsaturated diet expressed marginally less aminopyrine N-demethylase activity.The higher rates of hepatic glutathione S-transferase activities were observed in response to the 30% unsaturated diet.