This paper presents the results of animal tests and clinical studies on the metabolism of radioactive cyclophosphamide (Endoxan). The authors conclude that most of the metabolic breakdown of Endoxan occurs within the first two to three hours after administration. Further radiochromatographical investigations must be undertaken before the distribution of cytostatically active metabolites and their concentration in the tumor tissue can be elucidated. Studies on cerebral tumors have shown that the concentration is probably higher in the peripheral zones of the tumor than in the center.