Abstract: Groundnut oil was thermally oxidized at the temperature range of 180-200°C in open air for a period of 10 days at 4 hours per day. The extent of deterioration of thermoxidized groundnut oil were investigated spectroscopically, using infrared, ultra violet and atomic absorption spectroscopy and compared with fresh oil. The themoxidised oil samples differed in composition to the fresh oil, which served as control. Weanling rats were fed diet containing thermally oxidized groundnut oil while control rat were fed diet containing unheated fresh groundnut oil at 15% dietary level. The activity of Na+K+ ATPase were studied in the brain, liver, kidney, lungs and heart of experimental animals. The brain and the kidney had a relatively higher enzyme activity when compared to other tissues. Na+K+ ATPase activities of the liver, lungs and heart were not significantly affected (P> 0.05) while the ingestion of thermoxidised oil led to significant reduction (p< 005) of Na+K+ ATPase activity in the brain and kidney. It is considered that (a) thermoxidized oil causes reduced synthesis and structural changes in membrane phospholipids and (b) a probable impairment of kidney active transport and cerebral transmission of nerve impulse might have occurred due to reduced Na+K+ ATPase synthesis.