The distribution of tritiaded daunomycin and adriamycin have been studied in inbred C3H mice hearing spontaneous mammary carcinoma. It was found that adriamycin administration results in higher blood and tissue levels than daunomycin and that significant levels may be maintained in the tumor as much as a week after 3 injections of adriamycin 2.5 mg/kg at 12-intervals. Tumor hearing animals were treated with daunomycin or adriamycin daily or on alternate days. Daunomycin proved to be active only when administered daily, while adriamycin activity is marked even if administered every other day. Adriamycin activity is particularly marked if administered according to an intermittent schedule (weekly cycle of 3 injections every 12 h); on this schedule results the toxicity, as determined from weight loss and survival time of treated animals, is also somewhat lower.