This study examined the effect of metyrapone on the elimination rate of acetaminophen and on the apparent formation rate of acetaminophen metabolites in man. Metyrapone treatment, 1.5 g, increased the half-life of acetaminophen, decreased the fraction of the dose recovered in the urine as the glucuronide and increased the fraction of the dose recovered in urine as the sulfate and mercapturate conjugates. The apparent rate constant for the formation of acetaminophen glucuronide was significantly decreased by metyrapone while the apparent rate constants for the formation of the sulfate and mercapturic acid metabolites were unchanged or slightly increased, respectively. These data indicate that metyrapone inhibits acetaminophen glucuronidation and possibly enhances the oxidation of acetaminophen to its quantitatively minor yet highly toxic reactive metabolite. The extent to which the parallel pathways of acetaminophen elimination are also affected by inhibitors of cytochrome P-450-mediated oxidation will limit the efficacy of these types of potential antidotes for the treatment of acetaminophen overdose.