Histochemical observation of human breast tumors were caried out on lactic dehydrogenase, glucose-6-phosphate dehydrogenase, isocitric dehydrogenase, succinic dehydrogenase, malic dehydrogenase, glutamic dehydrogenase, α-glycerophosphate dehydrogenase and β-hydroxybutyric dehydrogenase. The surgically removed breast tumors were examined; mastopathia:, fibroadenoma:, gynecomastia:, and cancer:. The tumors were cut at 20μ in -20℃ cryostat, then the sections were stained by the methods described by Pearce (60)) with Nitro-BT as the electron acceptor. The following results were obtained. Lactic dehydrogenase activity was the strongest of all dehydrogenases studied in this report. It increased in proliferation, and slightly decreased in cancer. The stromal reaction was moderate. Glucose-6-phosphate dehydrogenase activity did not decreased in cancer tissues so much as that of other dehydrogenases. These results showed that anaerobic glycolysis and hexose monophosphate shunt played an important role in breast cancer metabolism. For investigation of tricarboxylic acid cycle, succinic dehydrogenase, isocitric dehydrogenase and malic dehydrogenase were studied. These three enzymes showed similar distribution to each other. Their activity were increased in proliferation, but decreased in cancer. Their activities were especially decreased in poorly differentiated cancer cells except for scirrhous carcinoma. Glutamic dehydrogenase activity revealed a simillar pattern to that of TCA cycle enzymes. As enzymes of fatty acid metabolisum, α-glycerohosphate dehydrogenase and β-hydroxybutyric dehydrogenase were examined. The activity was relatively weak in benign tumors and very weak in cancer.