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Simvastatin is a potent competitive inhibitor of the 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) which is the rate-limiting enzyme of cholesterol synthesis. In guinea-pigs, administration of a high oral dose of simvastatin (125 mg/kg/day at the beginning of the study) during 18 days had a major hepatotoxic effect whereas a lower oral dose (30 mg/kg/day) did not seem to cause any liver damage. A significant reduction in microsomal Cyt P 450 content was only observed on a high dose of simvastatin whereas HMG CoA reductase activity was reduced in the group with the low simvastatin dose. The hepatic microsomal aminopyrine N-demethylase activity remained unchanged in all groups. The liver lesion was hepatocellular necrosis accompanied in some animals by a biliary duct proliferation. It was associated with a 10-fold elevation in serum aspartate and alanine aminotransferase activities, as well as a great reduction in daily food intake and body weight (28%). The hepatotoxicity of simvastatin could result from the low basal content of HMG-CoA reductase in guinea-pig liver, the prolonged inhibition of mevalonate synthesis and probably, from the absence of HMG-CoA reductase enzyme de novo synthesis.