Colorimetoric estimation of succinic dehydrogenase activity by Triphenyl tetrazolium chloride was investigated by Kun and Abood. Following their method after some modifications, the author studied the enzymatic effects of cancer extracts as a study of cancer cachexia. Results obtained as follows:- 1) In healthy rats succinic dehydrogenase activity was higher in liver, heart and kidney homogenate, On the contrary, in spleen and lung tissue it was lower than in them. 2) In spleen, lung and kidney were no change in the dehydrogenase activity after the injection of cancer extracts in the abdominal cavity. Although liver tissue showed higher activity than before, the activity was gradualy decreased after the repeated injection. Studing the systematic effect of cancer extracts, succinic dehydrogenase activity is decreased after the injection of cancer extracts in the abdominal cavity.