Administration of pirprofen may produce microvesicular steatosis of the liver in humans. The effects of pirprofen on the mitochondrial beta-oxidation of fatty acids have been investigated in mice. In vitro, addition of 2 mM pirprofen decreased by 50% the formation of [14C]acid-soluble beta-oxidation products, and decreased by 70% the formation of [14C]CO2 upon incubation of hepatic mitochondria with [14C]palmitic acid, ATP, carnitine and coenzyme A. In vivo, administration of pirprofen (2 mmol . kg-1 i.p.), 1 hr before that of [U-14C]palmitic acid, decreased by 70% the exhalation of [14C]CO2 during the next 6 hr. Administration of pirprofen (2 mmol . kg-1 i.p.), 1 hr before the measurement, decreased plasma beta-hydroxybutyrate by 60%, plasma acetoacetate by 30% and blood glucose by 40%. Administration of pirprofen (2 mmol . kg-1 i.p.) 6 hr before sacrifice, doubled hepatic triglycerides content and produced microvesicular steatosis of the liver. We conclude that pirprofen inhibits the mitochondrial beta-oxidation of fatty acids in mice, thus explaining the microvesicular steatosis observed in mice and in some human subjects.