Young male rats were fed a balanced diet or a hyperlipidic diet (lard 30%, w/w) for 6 wk. After the first 2 wk each group was divided, one half being maintained as the control and the other half being fed the same diet with a polychlorinated biphenyl (Phénoclor DP6) added at a level of 50 mg/kg diet for 4 wk. In animals fed the balanced diet, DP6 increased the microsomal-protein and cytochrome P-450 content and the specific activity of the monooxygenases. The high-fat diet increased the inductive effect of DP6 on cytochrome P-450, aminopyrine demethylase and benzo[a]pyrene hydroxylase, but not on aniline hydroxylase or cytochrome c reductase.