Oral administrations of azoxybenzene (0, 25, 50 and 100 mg/kg b.w.) over a period up to 7 days caused a time and dose dependent decrease in the amount of cytochrome P-450 as well as in the ability of aminopyrine-N-demethylation and aniline hydroxylation in the hepatic microsomes of male and female rats. In both sexes there were retardations in the body weight gain and increases in the relative weight of the liver. The amount of the microsomal cytochrome b5 was increased.