AbstractThe effect of dietary lipids on hepatic and intestinal monooxygenases was studied by feeding C57BL/6N mice (for 2 wks) diets containing 5% and 23.5% (wt/wt) olive oil or corn oil. At the end of the feeding period, we measured arylhydrocarbon hydroxylase (AHH) activity in S9 preparations from liver, small intestine, and colon; and, using the same S9 preparations from the liver, we observed the activation of the following three dietary promutagens:2‐amino‐3‐methylimidazo(4,5‐f)quinoline,2‐amino‐3,8‐dimethylimidazo(4,5‐f) quinoxaline, and 2‐amino‐6‐methyldipyrido(l,2‐a:3’,2'‐d)imidazole. The results showed that high‐fat diets increased hepatic AHH activity both in corn oil and olive oil diets compared with the low‐fat diets; also, a 5% corn oil diet had significantly higher AHH activity compared with the 5% olive oil diet. AHH activity was, respectively, 48.6±5.1 and 79.5±11.4 pmol3OH‐benzo[a]pyrene formed/mg/min in the 5% and 23.5% olive oil diets and 66.1±5.1 and 83.9±12.2 in the 5% and 23.5% corn oil diets; values are means±SE, n = 16. The results also showed a significant increase in the ability of hepatic S9 fractions from animals on high‐fat diets to activate promutagens in the Salmonella/plate test. On the contrary, AHH activity in the small intestine and colon was not affected by the fat content of the diet.