Liver blood flow and indices of hepatic drug metabolism (antipyrine elimination rate and cytochrome P-450 concentration in liver biopsy specimens) were studied in 19 epileptics on long-term anticonvulsant treatment, and in 18 controls. The size of the liver and the total estimated liver blood flow were greater inthe epileptics than in the controls, whereas the relative liver blood flow (per unit weight of the liver) was not significantly different. The epileptics had higher cytochrome P-450 levels and they eliminated antipyrine faster than the controls. It was concluded that long-term ingestion of enzyme-inducing anticonvulsants is associated with an increase in the total hepatic blood flow in parallel with the increase in liver size, and not as an independent phenomenon. Since the relative perfusion rate of the hepatocytes was unchanged, the enhanced activity of drug metabolizing enzymes is presumed to be mainly responsible for the increased drug clearance observed in epileptic subjects.