Methacrylonitrile (MeAN) is a plastic monomer. Its effect on membrane bound enzymes like Na+K+ -ATPase, Ca2+ -ATPase, Mg2+ -ATPase, NADH dehydrogenase, alkaline phosphatase (ALP) and various elements like sodium (Na+), potassium (K+), and calcium (Ca2+) in rat brain were studied. Administration of 50 mg/kg body weight/day (0.25 LD50) and 100 mg/kg body weight/day (0.5 LD50) by gavage to rats for 7 days resulted in a significant decrease in activities of Na+K+ -ATPase, Ca2+ -ATPase, Mg2+ -ATPase, and NADH dehydrogenase. A significant reduction in calcium content, potassium content and a significant increase in sodium content and alkaline phosphatase activity in MeAN treated animals were observed. Inhibition of membrane bound enzymes occurred due to either direct effect of MeAN or indirect effect of changes in ionic homeostasis in MeAN treated animals.