In order to study influences of OX on the activity of succinic dehydrogenase in Yoshida sarcoma cells transplanted intramusculrally in the albino-rat thigh, the author injected OX intra-tumorally or intravenously into tumor bearing rats and carried out biochemical and histochemical investigations. The results are as follows: 1. In the cases given direct, intratumoral injection of OX, the activity of succinic dehydrogenase in Yoshida sarcoma cells thus transplanted is decreased, and it recovers very slowly but the same activity in muscle tissues recovers more rapidly. 2. In the histochemical observations when OX is injected intratumorally, central necrosis is increased and also a marked enlargement of bionecrotic foci occurs, but in the outer-most layer colonies of proliferative cells can be recognized. 3. When OX is injected intravenously into tumor bearing rats, the activity of the dephydrogenase is rather increased in tumor tissue, and the part where such increase occurred ultimately tends to become necrotic. In addition, the author discussed briefly about the relation between these changes in the activity of dehydrogenase and OX.