Dieldrin, an organochlorine insecticide, when administered orally to male albino rats at a dose of 5 mg/kg body weight/day for 15 days inhibits the enzyme Mg2+ -ATPase and stimulates the activity of 5 ' nucleotidase and NADH-dehydrogenase in liver plasma membrane. l-ascorbic acid supplementation offers some protection in relation to the inhibition of Mg2+ -ATPase. © 1982.