Reye syndrome, characterized by acute encephalopathy, selective liver damages, a fatty degeneration in visceral organs and miserable prognosis, is probably caused by various drugs, especially antipyretic such as acetylsalicylate. The incidence of the disease has been decreased by prohibition of administration of aspirin for children with high fever, especially caused by influenza infection in western countries, also in Japan. The pathophysiology of the disease has extensively studied, however, still being unknown to be dissolved. Our previous study of lipid analysis of brain from experimental measles encephalitis revealed an increase of cholesterol ester and decrease of sphingomyelin. As cholesterol ester is synthesized from cholesterol and fatty acids catalyzed by acylCoA-acyltransferase(ACAT), ACAT activity can be increased in the experimental animal brain. In the present report, ACAT m-RNA could not be expressed in control brain but in the experimental animal brain, so ACAT may play a role in pathogenesis of Reye syndrome.