The bipyridyl compound metyrapone is a potent inhibitor of cytochromes P450, a gene superfamily of haemoproteins involved in the metabolism of many xenobiotics as well as endogenous compounds such as steroid hormones. Administration of metyrapone to male rats induces the expression of the cytochrome P450 sub-family 3A (CYP3A). In order to determine whether metyrapone was causing the induction of CYP3A by blocking endogenous glucocorticoid metabolism, CYP3A levels were examined in rat hepatocytes cultured in serum-free medium supplemented with hydrocortisone 21-hemisuccinate plus or minus metyrapone. Western blotting indicated that metyrapone alone induces CYP3A and that hydrocortisone 21-hemisuccinate is ineffective. However, hydrocortisone 21-hemisuccinate enhanced the levels of CYP3A induced by metyrapone. In contrast, glucocorticoid-inducible tyrosine aminotransferase (TAT) activity was unaffected by metyrapone but metyrapone enhanced the levels induced by hydrocortisone 21-hemisuccinate. An examination of the metabolism of hydrocortisone by rat hepatocytes in vitro indicated that metyrapone perturbed the catabolism of hydrocortisone under conditions which give rise to an enhancement of hydrocortisone 21-hemisuccinate and hydrocortisone-dependent TAT induction. However, evidence is presented to suggest that such a perturbation of hydrocortisone metabolism could not account for the glucocorticoid potency amplifying property of metyrapone. Thus the induction of CYP3A and the enhancement of glucocorticoid-mediated TAT induction appears not to be associated with any perturbation in glucocorticoid metabolism but with some other as yet undefined mechanism(s).