Rats were pretreated with phenobarbital to induce hepatic cytochrome P-450. Compared to noninduced rats, a similar relation between the dose of acetaminophen and mortality, and between dose and changes in liver function (prothrombin index) and identical time courses, was found. The urinary excretion of acetaminophen mercapturate and acetaminophen cysteine was identical in induced and noninduced rats. The metabolism of acetaminophen in terms of blood levels and excreted metabolites was not influenced by phenobarbital induction. At the same dose level, hepatic necrosis was accelerated (maximum 24 h) compared to noninduced animals (maximum 72 h), but no difference in the maximum extent was found. These data cannot support the concept that induction of cytochrome P-450 leads to greater formation of the hypothetical toxic metabolite of acetaminophen, or that induction enhances its hepatotoxicity, in the rat.