In cholestatic rats, effects of phenobarbital or cholic acid on hepatic microsomal cytochrome p-450 and b₅ were investigated. The total contents of both cytochrome p-450 and cytochrome b₅ were decreased after bile duct ligation and the administration of estradiol. When cholic acid or phenobarbital was adminstrated in cholestatic rats, the decrease of cytochrome p-450 was prevented. The effects of cholic acid or phenobarbital on both ring-hydroxylation and N-hydroxylation of AAF in cholestatic rat hepatic microsomal fraction were studied. N-hydroxylation of AAF in bile duct ligated rat liver microsomes was reduced by 34%, but ring-hydroxylation was increased by 51%. In estradiol administrated rat, both ring-hydroxylation and N-hydroxylation of AAD was increased by 20 to 25%. In cholic acid administration, both ring-hydroxylation and N-hydroxylation was increased by about 10%. N-hydroxylation of AAF in phenobarbital treated rats was reduced more than ring-hydroxylation was reduced compared to the bile duct ligated group. Estradiol treated group which administrated with cholic acid or phenobarbital exhibited inhibited effects of N-hydroxylation.