The present study was undertaken to determine whether administration of adriamycin causes the depletion of riboflavin content. Rats received intraperitoneal injections of adriamycin (4 mg per kg body weight) for 6 consecutive days. Urinary riboflavin excretion began to increase after 2 days of treatment with adriamycin. Erythrocyte FAD levels decreased gradually and plasma lipid peroxide contents increased markedly at the 6th day. The activity coefficient of erythrocyte glutathione reductase showed a significant increase before the decrease of flavin content and the elevation of lipid peroxide level. Therefore, the value of this coefficient obtained from erythrocyte appears to be a reliable index of riboflavin deficiency, particularly during the early stage.