Metyrapone is known as an inhibitor of the oxidative drug metabolism in vitro. We have used the exhalation analysis as a tool to study the influence of this inhibitor on the demethylation of 14C-methacetin in vivo. In parallel we investigated the reductive metabolism of metyrapone in mice by measuring the concentrations of metyrapone and its reduced metabolite metyrapol with a HPLC-method. 50 mg of metyrapone/kg b. wt. resulted in a 90% inhibition of 14CO2 exhalation when given 2 min before the exhalation analysis was started. The prolongation of the intervals between i.p. metyrapone and substrate administration leads to a diminution of the in vivo inhibition. We found that the hepatic metyrapone concentration falls rapidly and passes the detection limit at 120 min. Transiently metyrapol reaches a maximal concentration 15 min after the application of metyrapone. The rapid reduction of metyrapone was confirmed in vitro with fresh mouse liver homogenates. The administration of metyrapol itself in vivo causes a decrease in 14CO2 exhalation, too. The 14CO2 exhalation curves after metyrapol correspond to the curves after metyrapone, when sufficient time was allowed for its reduction to metyrapol. It can be concluded that not only metyrapone itself but also its reductive metabolite metyrapol is an effective, however weaker inhibitor.