Semecarpus anacardium Linn. of the family Anacardiaceae has many applications in the Ayurvedic and Siddha systems of medicine. We have tested the antitumour activity of Semecarpus anacardium nut extract against experimental mammary carcinoma in animals. As there is a direct relationship between the proliferation of tumour cells and the activities of the glycolytic and gluconeogenic enzymes, we studied changes in the activities of enzymes involved in this metabolic pathway in the liver and kidney. The enzymes investigated were glycolytic enzymes, namely hexokinase, phosphoglucoisomerase, aldolase and the gluconeogenic enzymes, namely glucose-6-phosphatase and fructose-1,6-biphosphatase in experimental rats. A significant rise in glycolytic enzyme activities and a simultaneous fall in gluconeogenic enzyme activities were found in mammary carcinoma bearing rats. Drug administration returned these enzyme activities to their respective control activities.