Rats injected for 5–10 days with diphenyl-hydantoin (DPH) 75 mg/kg exhibit increased liver content of lactate dehydrogenase. However, the liver weight, the hepatic content of cytochrome P-450 and the microsomal ethanol oxidizing activity (MEOS) were unchanged. When rats were injected for 33–37 days with DPH (75 mg/ kg), the liver weight, the hepatic content of lactate dehydrogenase and cytochrome P-450 content, as well as hepatic MEOS activity, increased significantly. MEOS activity in the liver correlated significantly with lactate dehydrogenase activity (r = + 0.70) and cytochrome P-450 content (r = + 0.